2012ApJ...744..183K

PSR J1903+0327 is a millisecond pulsar with a mass of 1.67 M_☉ in a highly eccentric orbit (e = 0.44) around a main-sequence star. This unique system cannot be reconciled with current observations where millisecond pulsars are generally seen to orbit white dwarfs in almost exactly circular orbits. Current theoretical models of binary and stellar formation and evolution cannot explain the high eccentricity of this system either. In this work, we present three new epochs of optical spectroscopy for the companion to PSR J1903+0327, obtained to confirm the association of the main-sequence star with the pulsar. These three new epochs, together with the two previous ones, firmly establish the high eccentricity of the companion's orbit as predicted by pulsar timing. Using all five epochs of optical data, we have provided an independent estimate of the mass ratio, R = 1.56±0.15, as well as the systemic radial velocity of the binary, γ = 42.1±2.5 km/s. We constrain the spectral type of the pulsar companion to lie between F5 V and G0 V (a slightly earlier type than suggested previously) by measuring the equivalent widths of two of the three Ca-triplet lines (8498 Å, 8542 Å) and the Paschen line at 8598 Å (P14); we also broadly constrain the metallicity of the companion. Additionally, we have placed a somewhat better limit on the rotational velocity of the pulsar companion of v_rot sin i ≤ 66 km/s, which is still not sufficient to allow a test of general relativity using this system.