  PSR J2032+4127 is a gamma-ray and radio-emitting pulsar which has been
regarded as a young luminous isolated neutron star. However, its recent
spin-down rate has extraordinarily increased by a factor of two. We present
evidence that this is due to its motion as a member of a highly-eccentric
binary system with a 15-solar-mass Be star, MT91~213. Timing observations show
that, not only are the positions of the two stars coincident within 0.4 arcsec,
but timing models of binary motion of the pulsar fit the data much better than
a model of a young isolated pulsar. MT91~213, and hence the pulsar, lie in the
Cyg~OB2 stellar association, which is at a distance of only 1.4-1.7 kpc. The
pulsar is currently on the near side of, and accelerating towards, the Be star,
with an orbital period of 20-30 years. The next periastron is well-constrained
to occur in early 2018, providing an opportunity to observe enhanced
high-energy emission as seen in other Be-star binary systems.
