A Dual - Line Binary Radio Pulsarj

Binary radio pulsars are superb tools for mapping binary orbits, because of the precision of the pulse timing method (e.g Taylor & Weisberg 1989). To date, all orbital parameters for binary pulsars have been derived from observations of the pulsar alone. We present the rst observations of the radial velocity variations due to the binary motion of a companion to a radio pulsar. Our results demonstrate that the companion to the Small Magellanic Cloud pulsar PSR J0045{7319 is the B1V star identiied by Kaspi et al. (1994). The mass ratio of the system is 6:3 1:2, which, for a neutron star mass of 1.4 M , implies a mass of 8.8 1.8 M for the companion, consistent with the mass expected for a B1V star. The inclination angle for the binary system is therefore 44 5 , and the projected rotational velocity of the companion is 113 10 km s ?1. The heliocentric radial velocity of the binary system is consistent with that of other stars and gas in the same region of the Small Magellanic Cloud.