Most X-ray pulsars (over 35) are found in high mass X-ray binaries
(HMXBs), containing an early type (OB) star with a mass of
;SPMgt;5 M 
.
The number of X-ray pulsars that are not in HMXBs is very small, but recent
observations have virtually doubled this sample. This has also resulted from
the finding that a few optically unidentified sources have X-ray to optical
flux ratios incompatible with the presence of massive companions.
Several of these systems have low mass donor stars and, therefore, are
Low Mass X-ray Binaries (LMXBs).
Two of the LMXBs pulsars, Her X-1, which has a comparatively massive
companion of 
M , and GX 1+4,
whose companion is an M giant, are
peculiar systems (see, e.g., Rappaport & Joss 1983; Nagase 1989). The
remaining five LMXBs pulsars (4U 0142+614, 1E 1048.1-5937, 4U 1626-67,
RX J1838.4-0301 and 1E 2259+586) have very similar spin periods (P) in the
5 to 9 s range
(see Fig. A.5). This narrow period distribution is
remarkable, when one considers that the X-ray pulsars in HMXBs have
spin periods ranging from 69 ms (A 0538-66, Skinner et al. 1982) to
25 min (RX J0146.9+6121; Mereghetti, Stella & De Nile 1993). These LMXB
pulsars were proposed to belong to a homogeneous class of sources
and a possible explanation for their narrow spin period distribution was
suggested (Mereghetti & Stella 1995).