  W 601 (NGC 6611 601) is one of the handful of known magnetic Herbig Ae/Be
stars. We report the analysis of a large dataset of high-resolution
spectropolarimetry. The star is a previously unreported spectroscopic binary,
consisting of 2 B2 stars with a mass ratio of 1.8, masses of 12 M$_\odot$ and
6.2 $M_\odot$, in an eccentric 110-day orbit. The magnetic field belongs to the
secondary, W 601 B. The H$\alpha$ emission is consistent with an origin in W
601 B's centrifugal magnetosphere; the star is therefore not a classical Herbig
Be star in the sense that its emission is not formed in an accretion disk.
However, the low value of $\log{g} = 3.8$ determined via spectroscopic
analysis, and the star's membership in the young NGC 6611 cluster, are most
consistent with it being on the pre-main sequence. The rotational period
inferred from the variability of the H$\alpha$ line and the longitudinal
magnetic field $\langle B_z \rangle$ is 1.13 d. Modelling of Stokes $V$ and
$\langle B_z \rangle$ indicates a surface dipolar magnetic field $B_{\rm d}$
between 6 and $11$ kG. With its strong emission, rapid rotation, and strong
surface magnetic field, W 601 B is likely a precursor to H$\alpha$-bright
magnetic B-type stars such as $\sigma$ Ori E. By contrast, the primary is an
apparently non-magnetic ($B_{\rm d} < 300$ G) pre-main sequence early B-type
star. In accordance with expectations from magnetic braking, the non-magnetic
primary is apparently more rapidly rotating than the magnetic star.
