<PAGE>

                                                                    EXHIBIT 10.9
                                     NSTAR
                          Change in Control Agreement
                          ---------------------------

     AGREEMENT, made this 11th day of May, 1999, by and between Thomas J. May
("Executive") and NSTAR (the "Company").  This Agreement shall become effective
on the effective date (the "Effective Date") of the merger transaction between
Commonwealth Energy System and BEC Energy pursuant to the Amended and Restated
Agreement and Plan of Merger dated as of December 5, 1998 and amended and
restated as of May 4, 1999 among BEC Energy, Commonwealth Energy System, the
Company, BEC Acquisition, LLC and CES Acquisition, LLC.

                                 WITNESSETH

    WHEREAS, the Board of Trustees of the Company (the "Board") has determined
that it is in the best interests of the Company and its shareholders for the
Company to agree to provide benefits under the circumstances described below to
Executive and other executives who are responsible for the policy-making
functions of the Company and the overall viability of the Company's business;
and

    WHEREAS, the Board recognizes that the possibility of a change of control of
the Company is unsettling to such executives and desires to make arrangements at
this time to help assure their continuing dedication to their duties to the
Company and its shareholders, notwithstanding any attempts by outside parties to
gain control of the Company; and

    WHEREAS, the Board believes it important, should the Company receive
proposals from outside parties, to enable such executives, without being
distracted by the uncertainties of their own employment situation, to perform
their regular duties, and where appropriate to assess such proposals and advise
the Board as to the best interests of the Company and its shareholders and to
take such other action regarding such proposals as the Board determines to be
appropriate; and

    WHEREAS, the Board also desires to demonstrate to the executives that the
Company is concerned with their welfare and intends to provide that loyal
executives are treated fairly; and

    WHEREAS, the Board wishes to assure the executives of fair severance should
any of their employment terminate in specified circumstances following a change
of control of the Company and to assure the executives of other benefits upon a
change of control.

    NOW, THEREFORE, in consideration of the premises and the mutual covenants
contained herein, the parties hereto agree as follows:
<PAGE>

1.  In the event that any individual, corporation, partnership, company, or
other entity (a "Person"), which term shall include a "group" (within the
meaning of section 13(d) of the Securities Exchange Act of 1934 (the "Act")),
begins a tender or exchange offer, circulates a proxy to the Company's
shareholders, or takes other steps to effect a "Change of Control" (as defined
in Exhibit A attached hereto and made a part hereof), Executive agrees that he
will not voluntarily leave the employ of the Company and will render the
services contemplated in the recitals to this Agreement until such Person has
terminated the efforts to effect a Change of Control or until a Change of
Control has occurred.

2.  If, within 36 months following a Change of Control (the "Post Change of
Control Period") Executive's employment with the Company is terminated by the
Company for any reason other than for "Cause" or "Disability" (as defined
paragraph 4 below), or as a result of Executive's death, or Executive terminates
such employment for Good Reason (as defined in paragraph 5 below):

(1)  the Company will pay to Executive within 30 days of such termination of
     employment a lump-sum cash payment equal to the sum of (i) the Executive's
     annual base salary ("Annual Base Salary") through the date of such
     termination of employment to the extent not theretofore paid, (ii) a
     prorated portion of the target award payable under the Company's Executive
     Annual Incentive Compensation Plan, or any comparable or successor plan
     (the "Annual Plan") determined by calculating the product of (A) the target
     bonus award payable for the fiscal year in which the date of termination
     occurs under the Annual Plan, times (B) a fraction, the numerator of which
     is the number of days in the current fiscal year through the date of
     termination of employment, and the denominator of which is 365, (iii) a
     prorated portion of the target award payable under the Company's
     Performance Share Plan, or any comparable or successor plan (the "Long-Term
     Plan") for the performance period ending on the last day of the fiscal year
     during which the date of termination of employment occurs determined by
     calculating the product of (A) the target award payable for such
     performance period and (B) a fraction, the numerator of which is the number
     of days in the current performance period through the date of termination,
     and the denominator of which is the actual number of days in the
     performance period (provided that if any awards are expressed in shares of
     common stock rather than cash, the Company will pay the cash equivalent of
     such awards based on the closing price per share as reported in the Wall
     Street Journal (Eastern Edition) New York Stock Exchange Composite
     Transactions determined on the date prior to the date of the Change of
     Control or the average per share price for the 10 trading days preceding
     the date of the Change of Control (whichever is higher)) and (iv) any
     compensation for the fiscal year in which the date of termination occurs
     previously deferred by the Executive (together with any accrued interest or
     earnings thereon) and any accrued vacation pay, in each case to the extent
     not theretofore paid; and

(2)  any stock, stock option or cash awards granted to the Executive by the
     Company that would have become vested upon continued employment by the
     Executive

                                      -2-
<PAGE>

     shall immediately vest in full notwithstanding any provision to the
     contrary of such grant and shall remain exercisable until the earlier of
     the fifth anniversary of such termination and the latest date on which such
     grant could have been exercised; and

(3)  the Company will pay to Executive within 30 days of such termination of
     employment a lump-sum cash payment equal to three times:  (A) the amount of
     the Executive's Annual Base Salary at the rate in effect immediately prior
     to the date of termination or at the rate in effect immediately prior to
     the Change of Control, whichever is higher, and (B) the amount of the
     actual bonus paid to the Executive under the Annual Plan and the Long-Term
     Plan for the most recently completed fiscal year ended before the Change of
     Control, or the target bonus payable under the Annual Plan and Long-Term
     Plan for the fiscal year during which the termination of employment occurs,
     whichever is higher (provided that if any awards are expressed in shares of
     common stock rather than cash, the Company will pay the cash equivalent of
     such awards based on the closing price per share as reported in the Wall
     Street Journal (Eastern Edition) New York Stock Exchange Composite
     Transactions determined on the date prior to the date of the Change of
     Control or the average per share price for the 10 trading days preceding
     the date of the Change of Control (whichever is higher)); and

(4)  the Company will pay to Executive within 30 days of such termination of
     employment a lump-sum cash payment equal to the full balance standing to
     his credit with the Company under any and all deferred compensation plans
     or arrangements and the lump-sum actuarial equivalent of the Executive's
     accrued benefit under any supplement retirement plan or arrangement (the
     sum of the amounts described in subsections (a) and (d) shall be
     hereinafter referred to as the "Accrued Obligations"); and

(5)  an amount equal to the excess of (i) the lump-sum actuarial equivalent of
     the accrued benefit under (a) the Company's qualified defined benefit
     Retirement Plan (the "Retirement Plan") (utilizing actuarial assumptions no
     less favorable to the Executive than those in effect under the Retirement
     Plan immediately prior to the date of the Change of Control), and (b) any
     supplemental retirement plan of the Company in which the Executive
     participates (the "SERP") which the Executive would receive if the
     Executive's employment continued for three years after the date of
     termination assuming for these purposes that all accrued benefits are fully
     vested, and further assuming that the Executive's annual compensation for
     purposes of determining benefits under the Retirement Plan and SERP
     ("Covered Compensation") in each of the three years is at least equal to
     the higher of Executive's annual rate of Covered Compensation for the most
     recently completed fiscal year ending prior to the date of the Change of
     Control or the year in which the Change of Control occurs, over (ii) the
     lump-sum actuarial equivalent of the Executive's actual accrued benefit
     (paid or payable), if any, under the Retirement Plan and the SERP as of the
     date of termination; and

                                      -3-
<PAGE>

(6)  Executive, together with his dependents, will continue following such
     termination of employment to participate fully at the Company's expense in
     all welfare benefit plans, programs, practices and policies, including
     without limitation, life, medical, disability, dental, accidental death and
     travel insurance plans, maintained or sponsored by the Company immediately
     prior to the Change of Control, or receive substantially the equivalent
     coverage (or the full value thereof in cash) from the Company, until the
     longer of the third anniversary of such termination or any longer period as
     may be provided by the terms of the appropriate plan, program, practice or
     policy, provided, however, that if the Executive becomes re-employed with
     another employer and is eligible to receive medical or other welfare
     benefits under another employer-provided plan, the medical and other
     welfare benefits described herein shall be secondary to those provided
     under such other plan during such applicable period of eligibility.  For
     purposes of determining eligibility (but not the time of commencement of
     benefits) of the Executive for any retiree benefits pursuant to such plans,
     practices, programs and policies, the Executive shall be considered to have
     remained employed until three years after the date of termination and to
     have retired on the last day of such period; and

(7)  to the extent not theretofore paid or provided for, the Company shall
     timely pay or provide to the Executive any other amounts or benefits
     required to be paid or provided or which the Executive is eligible to
     receive under any plan, program, policy, practice, contract or agreement of
     the Company ("Other Benefits"); and

(8)  the Company will promptly reimburse Executive for any and all legal fees
     and expenses (including, without limitation, stenographer fees, printing
     costs, etc.) incurred by him as a result of such termination of employment,
     including without limitation all fees and expenses incurred to enforce the
     provisions of this Agreement or contesting or disputing that the
     termination of his employment is for Cause or other than for Good Reason
     (regardless of the outcome thereof).

    Notwithstanding anything herein to the contrary, to the extent that any
payment or benefit provided for herein is required to be paid or vested at any
earlier date under the terms of any plan, agreement or arrangement, such plan,
agreement or arrangement shall control.

3.  Death, Disability, Cause, Other Than For Good Reason.

(1)  Death.  If the Executive's employment shall terminate during the Post
     Change of Control Period by reason of the Executive's death, this Agreement
     shall terminate without further obligations to the Executive's legal
     representatives under this Agreement, other than for payment of Accrued
     Obligations and the timely payment or provision of Other Benefits.  Accrued
     Obligations shall be paid to the Executive's estate or beneficiary, as
     applicable, in a lump sum in cash within 30 days of death.

                                      -4-
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(2)  Disability.  If the Executive's employment is terminated during the Post
     Change of Control Period by reason of the Executive's Disability, this
     Agreement shall terminate without further obligations to the Executive
     other than for payment of Accrued Obligations and the timely payment or
     provision of Other Benefits.  Accrued Obligations shall be paid to the
     Executive in a lump sum in cash within 30 days of the date of termination
     of employment.  For purposes of this Agreement, "Disability" shall mean the
     absence of the Executive from the Executive's duties with the Company on a
     full-time basis for 180 consecutive business days as a result of incapacity
     due to mental or physical illness which is determined to be total and
     permanent by a physician selected by the Company or its insurers and
     acceptable to the Executive or the Executive's legal representative.  If
     the Company determines in good faith that the Disability of the Executive
     has occurred during the Post Change of Control Period, it may give the
     Executive written notice of its intention to terminate the Executive's
     employment.  In such event, the Executive's employment with the Company
     shall terminate effective on the 30th day after receipt of such notice by
     the Executive, provided that, within the 30 days of such receipt, the
     Executive shall not have returned to full-time performance of the
     Executive's duties.

(3)  Cause.  If the Executive's employment shall be terminated for Cause (as
     defined in Section 4 below) during the Post Change of Control Period, this
     Agreement shall terminate without further obligations to the Executive
     other than the obligation to pay the Executive (A) his Annual Base Salary
     through the date of termination, (B) the amount of any compensation
     previously deferred by the Executive, and (C) Other Benefits, in each case
     to the extent theretofore unpaid.  If the Executive voluntarily terminates
     employment during the Post Change of Control Period, excluding a
     termination for Good Reason, this Agreement shall terminate without further
     obligations to the Executive other than for Accrued Obligations and the
     timely payment or provisions of Other Benefits.

    In such case, all Accrued Obligations shall be paid to the Executive in a
lump sum in cash within 30 days of date of the termination of employment.

4.  "Cause" means only: (a) commission of a felony or gross neglect of duty by
the Executive which is intended to result in substantial personal enrichment of
the Executive at the expense of the Company, (b) conviction of a crime involving
moral turpitude, or (c) willful failure by the Executive of his duties to the
Company which failure is deliberate on the Executive's part, results in material
injury to the Company, and continues for more than 30 days after written notice
given to the Executive pursuant to a two-thirds vote of all of the members of
the Board at a meeting called and held for such purpose (after reasonable notice
to Executive) and at which meeting the Executive and his counsel were given an
opportunity to be heard, such vote to set forth in reasonable detail the nature
of the failure.  For purposes of this definition of Cause, no act or omission
shall be considered to have been "willful" unless it was not in good faith and
the Executive had knowledge at the time that the act or omission was not in the
best interest of the Company.  Any act, or failure to act, based on authority
given pursuant to a

                                      -5-
<PAGE>

resolution duly adopted by the Board or upon the instructions of the Chief
Executive Officer or another senior officer of the Company or based on the
advice of counsel of the Company shall be conclusively presumed to be done, or
omitted to be done, by the Executive in good faith and in the best interest of
the Company.

5.  Executive shall be deemed to have voluntarily terminated his employment for
Good Reason if the Executive leaves the employ of the Company for any reason
following:

(1)  The assignment to the Executive of any duties inconsistent in any respect
     with the Executive's position (including status, offices, titles and
     reporting requirements), authority, duties or responsibilities immediately
     prior to the Change of Control; or any other action by the Company which
     results in a diminution in such position, authority, duties or
     responsibilities, excluding for this purpose an isolated, insubstantial and
     inadvertent action not taken in bad faith and which is remedied by the
     Company promptly after receipt of notice thereof given by the Executive; or

(2)  Any reduction in the Executive's rate of Annual Base Salary for any fiscal
     year to less than 100% of the rate of Annual Base Salary paid to him in the
     completed fiscal year immediately preceding the Change of Control, or
     reduction in Executive's total cash and stock compensation opportunities,
     including salary and incentives, for any fiscal year to less than 100% of
     the total cash and stock compensation opportunities made available to him
     in the completed fiscal year immediately preceding the Change of Control
     (for this purpose, such opportunities shall be deemed reduced if the
     objective standards by which the Executive's incentive compensation
     measured become more stringent or the amount of such compensation is
     materially reduced on a discretionary basis from the amount that would be
     payable solely by reference to the objective standards); or

(3)  Failure of the Company to continue in effect any retirement, life, medical,
     dental, disability, accidental death or travel insurance plan, in which
     Executive was participating immediately prior to the Change of Control
     unless the Company provides Executive with a plan or plans that provide
     substantially similar benefits, or the taking of any action by the Company
     that would adversely effect Executive's participation in or materially
     reduce Executive's benefits under any of such plans or deprive Executive of
     any material fringe benefit enjoyed by Executive immediately prior to the
     Change of Control other than an isolated, insubstantial and inadvertent
     failure not occurring in bad faith and which is remedied by the Company
     promptly after receipt of notice thereof given by the Executive; or

(4)  The Company requires Executive to be based at any office or location
     outside the Greater Boston Metropolitan Area or the Company requires the
     Executive to

                                      -6-
<PAGE>

     travel on Company business to a substantially greater extent than required
     immediately prior to the date of Change of Control; or

(5)  Any purported termination by the Company of the Executive's employment
     otherwise than is expressly permitted by this Agreement; or

(6)  Any failure by the Company to comply with and satisfy Section 8 of this
     Agreement.

    For purposes of this Section 5, any good faith determination of Good Reason
made by the Executive shall be conclusive.

    6. If any payment or benefit received by Executive (whether paid or payable
or distributed or distributable pursuant to the terms of this Agreement or
otherwise, but determined without regard to any additional payments required
under this Section 6) would be subject to the excise tax imposed by Section 4999
of the Internal Revenue Code of 1986, as amended (the "Code"), or any interest
or penalties are incurred by the Executive with respect to such excise tax), the
Company will pay to Executive an additional amount in cash (the "Additional
Amount") equal to the amount necessary to cause the aggregate payments and
benefits received by Executive, including such Additional Amount (net of all
federal, state, and local income taxes and all taxes payable as a result of the
application of Sections 28OG and 4999 of the Code and including any interest and
penalties with respect to such taxes) to be equal to the aggregate payments and
benefits Executive would have received, excluding such Additional Amount (net of
all federal, state and local income taxes) as if Sections 28OG and 4999 of the
Code (and any successor provisions thereto) had not been enacted into law.


    Following the termination of Executive's employment, Executive may submit to
the Company a written opinion (the "Opinion") of a nationally recognized
accounting firm, employment consulting firm, or law firm selected by Executive
setting forth a statement and a calculation of the Additional Amount.  The
determination of such firm concerning the extent of the Additional Amount (which
determination need not be free from doubt), shall be final and binding on both
Executive and the Company.  The Company will pay to Executive the Additional
Amount not later than 10 days after such firm has rendered the Opinion.  The
Company agrees to pay the fees and expenses of such firm in preparing and
rendering the Opinion.

    If, following the payment to Executive of the Additional Amount, Executive's
liability for the excise tax imposed by Section 4999 of the Code on the payments
and benefits received by Executive is finally determined (at such time as the
Internal Revenue Service is unable to make any further adjustment to the amount
of such liability) to be less than the amount thereof set forth in the Opinion,
Executive shall reimburse the Company, without interest, in an amount equal to
the amount by which the Additional Amount should be reduced to reflect such
decrease in the actual excise tax liability.  The calculation of such
reimbursement shall be made by a nationally recognized accounting firm, an
employment consulting firm, or a law firm selected by Executive,

                                      -7-
<PAGE>

whose determination shall be binding on Executive and the Company and whose fees
and expenses therefore shall be paid by the Company.

    7. In the case of any dispute under this Agreement, Executive may initiate
binding arbitration in Boston, Massachusetts, before the American Arbitration
Association by serving a notice to arbitrate upon the Company or, at Executive's
election, institute judicial proceedings, in either case within 90 days of the
effective date of his termination or, if later, his receipt of notice of
termination, or such longer period as may be reasonably necessary for Executive
to take such action if illness or incapacity should impair his taking such
action within the 90-day period. The Company shall not have the right to
initiate binding arbitration, and agrees that upon the initiation of binding
arbitration by Executive pursuant to this paragraph 7 the Company shall cause to
be dismissed any judicial proceedings it has brought against Executive relating
to this Agreement. The Company authorizes Executive from time to time to retain
counsel of his choice to represent Executive in connection with any and all
actions, proceedings, and/or arbitration, whether by or against the Company or
any trustee, officer, shareholder, or other person affiliated with the Company,
which may affect Executive's rights under this Agreement. The Company agrees (i)
to pay the fees and expenses of such counsel, (ii) to pay the cost of such
arbitration and/or judicial proceeding, and (iii) to pay interest to Executive
on all amounts owed to Executive under this Agreement during any period of time
that such amounts are withheld pending arbitration and/or judicial proceedings.
Such interest will be at the base rate as announced from time to time by
BankBoston, N.A.

    In addition, notwithstanding any existing prior attorney-client relationship
between the Company and counsel retained by Executive, the Company irrevocably
consents to Executive entering into an attorney-client relationship with such
counsel and agrees that a confidential relationship shall exist between
Executive and such counsel.

    8. If the Company is at any time before or after a Change of Control merged
or consolidated into or with any other corporation or other entity (whether or
not the Company is the surviving entity), or if substantially all of the assets
thereof are transferred to another corporation or other entity, the provisions
of this Agreement will be binding upon and inure to the benefit of the
corporation or other entity resulting from such merger or consolidation or the
acquirer of such assets (the "Successor Entity"), and this paragraph 8 will
apply in the event of any subsequent merger or consolidation or transfer of
assets. The Company will require any such Successor Entity to assume expressly
and agree to perform this Agreement in the same manner and to the same extent
that the Company would be required to perform if no such transaction had taken
place. As used in this Agreement, "Company" shall mean the Company as
hereinbefore defined and any Successor Entity which assumes and agrees to
perform this Agreement by operation of law or otherwise.

    In the event of any merger, consolidation, or sale of assets described
above, nothing contained in this Agreement will detract from or otherwise limit
Executive's right to or privilege of participation in any stock option or
purchase plan or any bonus, profit sharing, pension, group insurance,
hospitalization, or other incentive or benefit plan or arrangement which may be
or

                                      -8-
<PAGE>

become applicable to executives of the corporation resulting from such merger
or consolidation or the corporation acquiring such assets of the Company.

    In the event of any merger, consolidation, or sale of assets described
above, references to the Company in this Agreement shall unless the context
suggests otherwise be deemed to include the entity resulting from such merger or
consolidation or the acquirer of such assets of the Company.

    9. Any termination by the Company for Cause, or by the Executive for Good
Reason, shall be communicated by Notice of Termination to the other party hereto
given in accordance with the last paragraph of Section 14 of this Agreement. For
purposes of this Agreement, a "Notice of Termination" means a written notice
which (i) indicates the specific termination provision in this Agreement relied
upon, (ii) to the extent applicable, sets forth in reasonable detail the facts
and circumstances claimed to provide a basis for termination of the Executive's
employment under the provision so indicated and (iii) if the Date of Termination
(as defined below) is other than the date of receipt of such notice, specifies
the termination date (which date shall be not more than thirty days after the
giving of such notice). The failure by the Executive or the Company to set forth
in the Notice of Termination any fact or circumstance which contributes to a
showing of Good Reason or Cause shall not waive any right of the Executive or
the Company, respectively, hereunder or preclude the Executive or the Company,
respectively, from asserting such fact or circumstance in enforcing the
Executive's or the Company's rights hereunder.

    "Date of Termination" means (i) if the Executive's employment is terminated
by the Company for Cause, or by the Executive for Good Reason, the date of
receipt of the Notice of Termination or any later date specified therein, as the
case may be, (ii) if the Executive's employment is terminated by the Company
other than for Cause or Disability, the Date of Termination shall be the date on
which the Company notifies the Executive of such termination and (iii) if the
Executive's employment is terminated by reason of death or Disability, the Date
of Termination shall be the date of death of the Executive or the effective date
of the Disability, as the case may be.

    10. All payments required to be made by the Company hereunder to Executive
or his dependents, beneficiaries, or estate will be subject to the withholding
of such amounts relating to tax and/or other payroll deductions as may be
required by law.

    11. There shall be no requirement on the part of the Executive to seek other
employment or otherwise mitigate damages in order to be entitled to the full
amount of any payments and benefits to which Executive is entitled under this
Agreement, and the amount of such payments and benefits shall not be reduced by
any compensation or benefits received by Executive from other employment.

    12. Nothing contained in this Agreement shall be construed as a contract of
employment between the Company and the Executive, or as a right of the Executive
to continue in the employ of the Company, or as a limitation of the right of the
Company to discharge the

                                      -9-
<PAGE>

Executive with or without Cause; provided that the Executive shall have the
right to receive upon termination of his employment the payments and benefits
provided in this Agreement and shall not be deemed to have waived any rights he
may have either at law or in equity in respect of such discharge.

    13. No amendment, change, or modification of this Agreement may be made
except in writing, signed by both parties.

    14. This Agreement shall terminate on the third anniversary of the Effective
Date, provided, however, that commencing on the date one year after the
Effective Date, and on each annual anniversary of such date (each such date
hereinafter referred to as a "Renewal Date"), unless previously terminated, the
term of this Agreement shall be automatically extended so as to terminate three
years from such Renewal Date, unless at least sixty days prior to the Renewal
Date the Company shall give notice to the Executive that the term of this
Agreement shall not be so extended. This Agreement shall not apply to a Change
of Control which takes place after the termination of this Agreement.

    Payments made by the Company pursuant to this Agreement shall be in lieu of
severance payments, if any, which might otherwise be available to Executive
under any severance plan, policy, program or arrangement generally applicable to
the employees of the Company.  If for any reason Executive receives severance
payments (other than under this Agreement) upon the termination of his
employment with the Company, the amount of such payments shall be deducted from
the amount paid under this Agreement.  The purpose of this provision is solely
to avert a duplication of benefits; neither this provision nor the provisions of
any other agreement shall be interpreted to reduce the amount payable to
Executive below the amount that would otherwise have been payable under this
Agreement.

    The provisions of this Agreement shall be binding upon and shall inure to
the benefit of Executive, his executors, administrators, legal representatives,
and assigns, and the Company and its successors.

    The validity, interpretation, and effect of this Agreement shall be governed
by the laws of The Commonwealth of Massachusetts.  Any ambiguities in this
Agreement shall be construed in favor of the Executive.

    The invalidity or unenforceability of any provisions of this Agreement shall
not affect the validity or enforceability of any other provision of this
Agreement, which shall remain in full force and effect.

    The Company shall have no right of set-off or counterclaims, in respect of
any claim, debt, or obligation, against any payments to Executive, his
dependents, beneficiaries, or estate provided for in this Agreement.

    No right or interest to or in any payments shall be assignable by the
Executive; provided, however, that this provision shall not preclude him from
designating one or more beneficiaries to

                                      -10-
<PAGE>

receive any amount that may be payable after his death and shall not preclude
the legal representative of his estate from assigning any right hereunder to the
person or persons entitled thereto under his will or, in the case of intestacy,
to the person or persons entitled thereto under the laws of intestacy applicable
to his estate. The term "beneficiaries" as used in this Agreement shall mean a
beneficiary or beneficiaries so designated to receive any such amount, or if no
beneficiary has been so designated, the legal representative of the Executive's
estate.

    No right, benefit, or interest hereunder shall be subject to anticipation,
alienation, sale, assignment, encumbrance, charge, pledge, hypothecation, or
set-off in respect of any claim, debt, or obligation, or to execution,
attachment, levy, or similar process, or assignment by operation of law.  Any
attempt, voluntary or involuntary, to effect any action specified in the
immediately preceding sentence shall, to the full extent permitted by law, be
null, void, and of no effect.

    All notices and other communications hereunder shall be in writing and shall
be given by hand delivery to the other party or by registered or certified mail,
return receipt requested, postage prepaid, addressed as follows:


    If to the Executive:  Thomas J. May
                          NSTAR
                          800 Boylston Street
                          Boston, MA  02199

    If to the Company:    NSTAR
                          800 Boylston Street
                          Boston, MA  02199
                          Attention: General Counsel

or to such other address as either party shall have furnished to the other in
writing in accordance herewith.  Notice and communications shall be effective
when actually received by the addressee.

    15.  Upon the Effective Date, this Agreement will supercede and replace the
Change in Control Agreement dated as of July 8, 1996 by and between the
Executive and Boston Edison Company.

    IN WITNESS WHEREOF, NSTAR and Executive have each caused this Agreement to
be duly executed and delivered as of the date set forth above.

                         NSTAR


                         By: /s/ R. D. Wright
                             ------------------
                             Name: R. D. Wright
                             Title: President


                             /s/ Thomas J. May
                             ------------------
                             Thomas J. May

                                      -11-
<PAGE>

                                 EXHIBIT A

    Change of Control.  For the purposes of this Agreement, a "Change of
    -----------------
Control" shall mean:

(1)  The acquisition by any Person of beneficial ownership (within the meaning
     of Rule 13d-3 promulgated under the Exchange Act) of 30% or more of either
     (i) the then outstanding shares of common stock of the Company (the
     "Outstanding Company Common Stock") or (ii) the combined voting power of
     the then outstanding voting securities of the Company entitled to vote
     generally in the election of trustees (the "Outstanding Company Voting
     Securities"); provided, however, that for purposes of this subsection (a),
     the following acquisitions shall not constitute a Change of Control:  (i)
     any acquisition directly from the Company, (ii) any acquisition by the
     Company, (iii) any acquisition by any employee benefit plan (or related
     trust) sponsored or maintained by the Company or any corporation controlled
     by the Company or (iv) any acquisition by any corporation pursuant to a
     transaction which complies with clauses (i), (ii) and (iii) of subsection
     (c) of this Exhibit A; or

(2)  Individuals who, as of the date hereof, constitute the Board (the
     "Incumbent Board") cease for any reason to constitute at least a majority
     of the Board; provided, however, that any individual becoming a trustee
     subsequent to the date hereof whose election, or nomination for election by
     the Company's shareholders, was approved by a vote of at least a majority
     of the trustees then comprising the Incumbent Board shall be considered as
     though such individual were a member of the Incumbent Board, but excluding,
     for this purpose, any such individual whose initial assumption of office
     occurs as a result of an actual or threatened election contest with respect
     to the election or removal of trustees or other actual or threatened
     solicitation of proxies or consents by or on behalf of a Person other than
     the Board; or

(3)  Consummation of a reorganization, merger or consolidation or sale or other
     disposition of all or substantially all of the assets of the Company (a
     "Business Combination"), in each case, unless, following such Business
     Combination, (i) all or substantially all of the individuals and entities
     who were the beneficial owners, respectively, of the Outstanding Company
     Common Stock and Outstanding Company Voting Securities immediately prior to
     such Business Combination beneficially own, directly or indirectly, more
     than 50% of, respectively, the then outstanding shares of common stock and
     the combined voting power of the then outstanding voting securities
     entitled to vote generally in the election of directors, as the case may
     be, of the corporation resulting from such Business Combination (including,
     without limitation, a corporation which as a result of such transaction
     owns the Company or all or substantially all of the Company's assets either
     directly or through one or more subsidiaries) in substantially the same
     proportions as their ownership, immediately prior to such Business
     Combination of the Outstanding Company Common Stock and outstanding Company
     Voting

                                      -12-
<PAGE>

     Securities, as the case may be, (ii) no Person (excluding any corporation
     resulting from such Business Combination or any employee benefit plan (or
     related trust) of the Company or such corporation resulting from such
     Business Combination) beneficially owns, directly or indirectly, 30% or
     more of, respectively, the then outstanding shares of common stock of the
     corporation resulting from such Business Combination or the combined voting
     power of the then outstanding voting securities of such corporation except
     to the extent that such ownership existed prior to the Business Combination
     and (iii) at least a majority of the members of the board of directors of
     the corporation resulting from such Business Combination were members of
     the Incumbent Board at the time of the execution of the initial agreement,
     or of the action of the Board, providing for such Business Combination; or

(4)  Approval by the shareholders of the Company of a complete liquidation or
     dissolution of the Company.

                                      -13-