Binary pulsar with a very small mass function

Much of the interesting physics concerning neutron stars and their evolution depends for its experimental foundation on observations of the rotation rates of pulsars. To continue recent efforts of our group in this area^1-3, we began a series of pulse-arrival-time observations of ~70 pulsars in January 1985. Most of the pulsars in this study were discovered in the Princeton/NRAO pulsar survey of the preceding two years^4,5. Soon after we began these observations it became clear that PSR2303+46 was a binary pulsar²; it is now evident that PSR1831-00 is also a member of a binary system, the seventh such radio pulsar known. It moves in an orbit with a period of 1.81 days, a small eccentricity, and an unusually small mass function of 0.00012 Msolar (where Msolar is the mass of the Sun). With a period P = 0.521 s and period derivative P<~ 10^-17 s s^-1, PSR1831-00, like the other known binary pulsars, has a relatively weak magnetic field. We discuss the features of this system that provide clues to its evolutionary history and outline possible models for its formation.