A new class of X-ray Pulsars ?
  While the distribution of spin periods of High Mass X-ray Binaries (HMXBs) spans more than four orders of magnitude (69ms - 25min), the few known X-ray pulsars accreting from very low mass companions (<1 Msun) have very similar periods between 5.4 and 8.7s . These pulsars display also several other similarities and they are probably members of a subclass of Low Mass X-ray Binaries (LMXBs) with similar magnetic field (a few 10^11 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 10^35 erg/s. These pulsars might represent the closest members of a subclass of LMXBs characterized by lower luminosities, higher magnetic fields and smaller ages than non-pulsating LMXBs. 
L. Stella, S. Mereghetti and G. L. Israel