THE VERY-LOW MASS X-RAY BINARY PULSARS - A NEW CLASS OF SOURCES

While the distribution of spin periods of high-mass X-ray binaries spa ns more than four orders of magnitude (69 ms-25 minutes) the few known X-ray pulsars accreting from very low mass companions(<1 M.) have ver y similar periods between 5.4 and 8.7 s. These pulsars also display se veral other similarities, and we propose that they are members of a su bclass of low-mass X-ray binaries (LMXBs) with similar magnetic field (a few times 10(11) G), companion stars and, possibly, evolutionary hi stories. If they are rotating at, or close to, the equilibrium period, their properties are consistent with luminosities of the order of a f ew times 10(35) ergs s(-1). These pulsars might represent the closest members of a subclass of LMXBs characterized by lower luminosities, hi gher magnetic fields, and smaller ages than nonpulsating LMXBs.