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Exhibit 10.10

                             ARBINET HOLDINGS, INC.
                             STOCK OPTION AGREEMENT


            STOCK OPTION AGREEMENT (the "Agreement"), dated as of January 1,
2000, between ARBINET HOLDINGS, INC., a Delaware corporation (the "Company"),
and Robert S. Vaters (the "Optionee").

            The Company has engaged the Optionee as Executive Vice President and
Chief Financial Officer of the Company pursuant to an employment agreement dated
as of December 27, 1999 (the "Employment Agreement").

            The board of directors of the Company (the "Board") has established
the Company's Amended and Restated 1997 Stock Incentive Plan (the "Plan") in
order to provide certain individuals with a favorable opportunity to acquire
shares of the Company's common stock, par value $.001 per share ("Common
Stock").

            The Board has determined that the objectives of the Plan will be
furthered by granting to the Optionee a stock option pursuant to the Plan.

            In consideration of the foregoing and of the mutual undertakings set
forth in this Agreement, the Company and the Optionee agree as follows:

            SECTION 1. Definitions and Incorporation. Unless otherwise defined
herein, the terms used in the Agreement shall have the meanings given to such
terms in the Plan. The Plan is hereby incorporated in and made a part of this
Agreement as if fully set forth herein. The Optionee hereby acknowledges that he
or she has received a copy of the Plan.

            SECTION 2. Grant of Option; Option Price. Pursuant to the Plan, the
Company hereby grants to the Optionee a stock option (the "Option") to purchase
an aggregate of 350,000 shares (the "Shares") of Common Stock. Each installment
of the Option shall be exercisable at a price per Share as set forth in Schedule
1 attached hereto and hereby made a part hereof (the "Exercise Price"). The
Option issued under this Agreement is intended to qualify as an Incentive Stock
Option ("ISO") under Section 422 of the Internal Revenue Code of 1986, as
amended. Nevertheless, to the extent that it exceeds the $100,000 rule of Code
Section 422(d), or to the extent the Option does not meet the ISO rules for some
other reason, this Option shall be treated as a Non-Qualified Stock Option
("NSO").

            SECTION 3. Exercisability. Subject to the further terms of this
Agreement and the Plan, the right to exercise the Option shall vest in
accordance with Schedule 1 attached hereto and hereby made a part hereof.

            Any Option which is not exercised in full on the applicable vesting
date for such Option shall not lapse but shall remain outstanding as to the
unexercised portion

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and shall continue in effect throughout remainder of the option term, except to
the extent otherwise provided in this Agreement, the Plan or Schedule 1.

            Unless earlier terminated pursuant to the provisions of the Plan,
the unexercised portion of the Option shall expire and cease to be exercisable
at 12:01 a.m. on the date which is the fifth anniversary of the date hereof (the
"Expiration Date").

            SECTION 4. Method of Exercise. The Option or any part thereof may be
exercised by delivery of a Notice of Exercise substantially in the form attached
hereto and as provided in Article II of the Plan.

            SECTION 5. Upon termination of the Optionee's employment with or, as
the case may be, Optionee's association with the Company or a subsidiary or as a
director, consultant or otherwise, by reason of dismissal for cause, the Option
shall terminate and expire. As used herein the term "cause" shall have the
meaning set forth in Section 1.6.5 of the Plan.

            SECTION 6. Investment Representations. The Optionee hereby
represents and warrants to and agrees with the Company, as of the date hereof,
as follows:

            6.1 Review of Documents. The Optionee acknowledges that the Optionee
has been granted access to and has reviewed carefully the Plan and, before any
exercise of the Option, the Optionee will review such records of the Company as
may be necessary to permit the Optionee to evaluate the merits and risks of an
investment in Common Stock. The Optionee is entering into this Agreement and the
transactions contemplated hereby solely in reliance on the Optionee's own
investigation and such review. The Optionee has had an opportunity to meet with
the officers of the Company subsequent to review of such information to discuss
with them the Optionee's questions concerning the Company and the terms and
conditions of the acquisitions hereunder.

            6.2 Acquisition of Shares for Own Account. The Optionee will acquire
the Shares, if at all, pursuant to this Agreement with the Optionee's own funds,
and not with the funds of anyone else. The Shares will be acquired, if at all,
for the Optionee's own account, not as a nominee or agent and not for the
account of any other person or firm. No one else has or will have on any
exercise of the Option any interest, beneficial or otherwise in the Shares to be
acquired on such exercise. The Optionee is not, and prior to any exercise of the
Option will not be, obligated to transfer the Shares or any interest therein to
anyone else and the Optionee does not and will not have any agreement or
understanding to do so. The Optionee will acquire the Shares, if at all, for
investment for an indefinite period and not with a view to the sale or
distribution of any part or all thereof, by public or private sale or other
disposition, and has no intention of selling, granting any participation in or
otherwise distributing or disposing of the Shares or any interest therein. The
Optionee does not, and on any exercise of the Option will not, intend to
subdivide the Optionee's acquisition of the Shares with anyone.

            6.3 Restricted Nature of Shares. The Optionee is (or will be at the
time of any acquisition of the Shares) able to bear the economic risk of the
investment in such Shares and is aware that the Optionee must be prepared to
hold the Shares received for an indefinite period and that such Shares have not
been registered under the Securities Act of 1933, as amended (the "Act") or any
state securities laws, on the ground that no


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distribution or public offering of the Share is to be effected and such Shares
will be issued, if at all, by the Company in connection with transactions not
involving any public offering within the meaning of Section 4(2) of the Act. The
Optionee understands that the Company is relying in part on the Optionee's
representations as set forth herein for purposes of claiming the exemption from
registration under the Act provided by Section 4(2) thereof and that the basis
for such exemption may not be present if, notwithstanding the Optionee's
representations herein, the Optionee has in mind merely acquiring the Shares for
resale on the occurrence or nonoccurrence of some predetermined event. The
Optionee has no such intention.

            6.4 Information about the Company. The Optionee has carefully
reviewed and will carefully review all the information regarding the Company
access to which has been and will be accorded to the Optionee hereunder, is
thoroughly familiar with the business, operations and properties of the Company
by virtue of such review and of the Optionee's relationship with the Company as
a key employee thereof and has discussed with the officers of the Company any
questions the Optionee may have had with respect to the Company.

            6.5 Restrictions on Issuance and on Resales. (a) Optionee
acknowledges and agrees that if the Committee shall at any time determine that
any consent (as defined in Section 3.3.2 of the Plan) is necessary or desirable
as a condition of, or in connection with any exercise of the Option (any such
action being hereinafter referred to as a "plan action"), then such exercise
shall not be effective, in whole or in part, unless and until such consent shall
have been effected or obtained to the full satisfaction of the Committee.

            (b) Without in any way limiting the Optionee's representations as
set forth herein, the Optionee further agrees that the Optionee shall in no
event make any disposition of all or any part of or interest in the Shares and
that such Shares shall not be encumbered, pledged, hypothecated, sold or
transferred by the Optionee nor shall the Optionee receive any consideration for
such Shares or for any interest therein from any person, unless and until prior
to any proposed transfer, encumbrance, disposition, pledge, hypothecation or
sale of any Shares, either (i) a registration statement on Form S-1 or S-8 (or
any other form replacing such form or appropriate for the purpose under the Act)
with respect to such share proposed to be transferred or otherwise disposed of
shall be then effective or (ii) (x) the Optionee shall have notified the Company
of the proposed disposition and shall have furnished the Company with a detailed
statement of the circumstances surrounding the proposed disposition, (y) the
Optionee shall have furnished the Company with an opinion of counsel in form and
substance satisfactory to the Company to the effect that such disposition will
not require registration of such Shares under the Act or qualification of any
such shares under any other securities law and (z) such opinion of counsel shall
have been concurred in by counsel for the Company and the Company shall have
advised the Optionee of such concurrence.

            6.6 Shares Will Be "Restricted Securities". The Shares, if and when
issued, will be "restricted securities," as that term is defined in Rule 144
under the Act, and, accordingly, the Shares must be held indefinitely unless
they are subsequently registered under the Act or an exemption from such
registration is available. The Optionee understands and agrees that the Company
is not under any obligation to register the Shares under the Act or to comply
with Regulation A or any other exemption.


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            6.7 Company May Refuse to Transfer. Notwithstanding the foregoing,
in the event that the Optionee has, in the opinion of counsel for the Company,
acted in any manner not consistent with the representations and agreements of
the Optionee in this Agreement, the Company may refuse to transfer the
Optionee's Shares until such time as counsel for the Company is of the opinion
that such transfer will not require registration of any Shares under the Act or
qualification of any Shares under any other securities law.

            6.8 Indemnification. The Optionee hereby agrees to indemnify the
Company and hold it harmless from and against any and all liability, damage,
cost or expense incurred on account of or arising out of (a) any inaccuracy in
any of the representations, warranties or agreements set forth in this Section
6, (b) the disposition of any Share which the Optionee may receive, contrary to
the Optionee's representations, warranties and agreements set forth in this
Section 6 or (c) any action, suit or proceeding based on a claim that said
representations, warranties or agreements were inaccurate or misleading or
otherwise cause for obtaining damages or redress from the Company under the Act.

            6.9 Certificates Representing Shares to Be Legended. The Optionee
understands and agrees that certificates representing any Shares received on
exercise of the Option may bear a legend on the face thereof (or on the reverse
thereof with a reference to such legend on the face thereof) substantially in
the form set forth below, which legend restricts the sale, transfer or
disposition of the Share otherwise than in accordance with this Agreement:

                THE SHARES OF COMMON STOCK REPRESENTED BY THIS CERTIFICATE HAVE
                NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS
                AMENDED, AND MAY NOT BE ENCUMBERED, PLEDGED, HYPOTHECATED, SOLD,
                TRANSFERRED OR OTHERWISE DISPOSED OF IN THE ABSENCE OF AN
                EFFECTIVE REGISTRATION STATEMENT UNDER SAID ACT OR AN OPINION OF
                COUNSEL SATISFACTORY TO THE CORPORATION AND CONCURRED IN BY THE
                CORPORATION'S COUNSEL THAT SUCH REGISTRATION IS NOT REQUIRED
                UNDER SAID ACT OR SUCH TRANSACTION COMPLIES WITH RULES
                PROMULGATED BY THE SECURITIES AND EXCHANGE COMMISSION UNDER SAID
                ACT. IN ADDITION, SALE, TRANSFER, ENCUMBRANCE, HYPOTHECATION,
                GIFT OR OTHER DISPOSITION OR ALIENATION OF SUCH SHARES OR ANY
                INTEREST THEREIN IS RESTRICTED BY AND SUBJECT TO A STOCK OPTION
                AGREEMENT DATED AS OF DECEMBER 30, 1999, A COPY OF WHICH MAY BE
                INSPECTED AT THE PRINCIPAL OFFICE OF THE CORPORATION AND ALL OF
                THE PROVISIONS OF WHICH ARE INCORPORATED BY REFERENCE IN THIS
                CERTIFICATE AND MAY ADDITIONALLY BE SUBJECT TO THE RESTRICTIONS
                SET FORTH IN A STOCKHOLDERS AGREEMENT BETWEEN THE HOLDER AND THE
                COMPANY.


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            SECTION 7. Plan Provisions to Prevail. This Agreement is subject to
all of the terms and provisions of the Plan. Without limiting the generality of
the foregoing, by entering into this Agreement the Optionee agrees that no
member of the Committee shall be liable for any action or determination made in
good faith with respect to the Plan or any award thereunder or this Agreement.
In the event that there is any inconsistency between the provisions of this
Agreement and of the Plan, the provisions of the Plan shall govern.

            SECTION 8. Notices. Any notice to be given to the Company hereunder
shall be in writing and shall be addressed to Arbinet Holdings, Inc., 33
Whitehall Street, 19th Floor, New York, New York 10004, or at such other address
as the Company may hereafter designate to the Optionee by notice as provided in
this Section 8. Any notice to be given to the Optionee hereunder shall be
addressed to the Optionee at the address set forth beneath such Optionee's
signature hereto, or at such other address as the Optionee may hereafter
designate to the Company by notice as provided herein. A notice shall be deemed
to have been duly given when personally delivered or mailed by registered or
certified mail to the party entitled to receive it.

            SECTION 9. Successors and Assigns. This Agreement shall be binding
upon and inure to the benefit of the parties hereto and the successors and
assigns of the Company and, to the extent consistent with Section 5 of this
Agreement and with the Plan, the heirs and personal representatives of the
Optionee.

            SECTION 10. Governing Law. This Agreement shall be interpreted,
construed and administered in accordance with the laws of the State of Delaware.

            SECTION 11. Entire Agreement. This Agreement (subject to the
provisions of the Plan) contains the entire agreement among the parties relating
to the subject matter hereof and there are no other or further agreements
outstanding not specifically mentioned herein; provided, however, that the
parties may by agreement amend and supplement this Agreement in writing from
time to time as provided herein. The parties agree that all options to purchase
shares of capital stock of the Company granted to Optionee prior to the date
hereof have expired and shall be deemed canceled and are of no further force or
effect.


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            IN WITNESS WHEREOF, the parties hereto have executed this Agreement
as of the date and year first written above.

                                        ARBINET HOLDINGS, INC

                                        By:
                                           ----------------------------
                                           Name:
                                           Title:



                                        -------------------------------
                                        Robert S. Vaters

                                        Address:


                                        -------------------------------
                                        Social Security Number


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                                   SCHEDULE 1
                                RIGHT TO EXERCISE

      1. Subject to the conditions set forth in this Agreement, the right to
exercise the Option with respect to 1/48th of the Shares shall vest on the last
day of each full month that the Optionee is employed by the Company (the
"Vesting Dates"); provided that the Optionee is employed by the Company on the
applicable Vesting Dates. The per share Exercise Price with respect to the
Shares shall be $.254.

      2. Subject to the conditions set forth in this Agreement, notwithstanding
any provision herein to the contrary, (i) if an initial public offering of the
Common Stock (the "IPO") occurs during the first year of the Optionee's
employment, vesting shall accelerate to effect 12 months of vesting as of the
end of the month in which the IPO closing occurs, and unvested Options shall
vest 1/36th per month over the 36 months after the month in which the IPO
closing occurs; and (ii) all Options shall vest immediately prior to a "change
in control" event (which will not include an IPO or a private equity financing
transaction). For purposes hereof, a "change in control" shall have occurred if
the Company consummates (a) a merger or consolidation of the Company with any
other company (other than a wholly-owned subsidiary of the Company), other than
(i) a merger or consolidation which would result in the voting securities of the
Company outstanding immediately prior thereto continuing to represent (either by
remaining outstanding or by being converted into voting securities of the
surviving entity) 50% or more of the combined voting power of voting securities
of the Company or such surviving entity outstanding immediately after such
merger or consolidation (ii) a merger or consolidation affected to implement a
recapitalization of the Company (or similar transaction), (b) the sale of 50% or
more of the voting securities of the Company in a single transaction or a series
of related transactions following an IPO; or (c) a plan of complete liquidation
of the Company or an agreement for the sale or disposition by the Company of all
or substantially all of the Company's assets.


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                           FORM OF NOTICE OF EXERCISE

TO:     ARBINET HOLDINGS, INC.
        33 Whitehall Street
        19th Floor
        New York, New York 10004

FROM:   Robert S. Vaters

RE:     Notice of Exercise of Stock Option

DATE:   __________________________________

This letter shall constitute the written notice of exercise in accordance with
the terms of the Stock Option Agreement dated as of December 30, 1999 (the
"Stock Option Agreement") between Arbinet Holdings, Inc. (the "Company") and me,
required by terms of the Stock Option Agreement.

I hereby advise the Company that by means of this notice of exercise, I wish to
purchase, effective as of the date hereof, __________ shares of Common Stock at
an exercise price of $0.254 per share under the Stock Option Agreement.

(please check appropriate box below)

[ ] I am enclosing a check for a total of $_________ for such shares.

[ ] I am exercising the Option (as defined in the Stock Option Agreement) on a
    "cashless" basis as provided in Section 2.4.3 of the Plan (as defined in the
    Stock Option Agreement).

I authorize the certificate representing said shares to be issued in the name(s)
of:

Name _____________________________________________________________

Address __________________________________________________________

        __________________________________________________________

Social Security Number ___________________________________________
(May be the name and address of Undersigned or Undersigned and Spouse.)


                         __________________________________
                               OPTIONEE (signature)

Receipt of Notice of Exercise is hereby acknowledged on ____________________

                               ARBINET HOLDINGS, INC.

                               By:____________________________
                               Name: _________________________
                               Title:  Secretary


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