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 spans more than four orders of magnitude (69 ms-25 minutes) the few known X-ray pulsars accreting from very low mass companions (less than 1 solar mass) have very similar periods between 5.4 and 8.7 s. These pulsars also display several other similarities, and we propose that they are members of a subclass of low-mass X-ray binaries (LMXBs) with similar magnetic field (a few times 10<SUP>11</SUP> G), companion stars and, possibly, evolutionary histories. If they are rotating at, or close to, the equilibrium period, their properties are consistent with luminosities of the order of a few times 10<SUP>35</SUP> ergs/s. These pulsars might represent the closest members of a subclass of LMXBs characterized by lower luminosities, higher magnetic fields, and smaller ages than nonpulsating LMXBs.