  (abridged) PSR J1903+0327, a millisecond pulsar in an eccentric (e = 0.44)
95-day orbit with a (~ 1Msun) companion poses a challenge to our understanding
of stellar evolution in binary and multiple-star systems. Here we describe
optical and radio observations which rule out most of the scenarios proposed to
explain formation of this system. Radio timing measurements of three
post-Keplerian effects yield the most precise measurement of the mass of a
millisecond pulsar to date: 1.667 +/- 0.021 solar masses (99.7% confidence
limit) (...). Optical spectroscopy of a proposed main sequence counterpart show
that its orbital motion mirrors the pulsar's 95-day orbit; being therefore its
binary companion (...) The optical detection also provides a measurement of the
systemic radial velocity of the binary; this and the proper motion measured
from pulsar timing allow the determination of the systemic 3-D velocity in the
Galaxy. We find that the system is always within 270 pc of the plane of the
Galaxy, but always more than 3 kpc away from the Galactic centre. Thus an
exchange interaction in a dense stellar environment (like a globular cluster or
the Galactic centre) is not likely to be the origin of this system. We suggest
that after the supernova that formed it, the neutron star was in a tight orbit
with a main-sequence star, the present companion was a tertiary farther out.
The neutron star then accreted matter from its evolving inner MS companion,
forming a millisecond pulsar. The former donor star then disappears, either due
to a chaotic 3-body interaction with the outer star (caused by the expansion of
the inner orbit that necessarily results from mass transfer), or in the case of
a very compact inner system, due to ablation/accretion by the newly formed
millisecond pulsar.
