The orbital periods of low-mass X-ray binaries.

The evidence that LMXRB contain thick azimuthally structured accretion disks is discussed and the ways in which these can give rise to observable orbital modulations are presented. The orbital period distribution of LMXRBs is shown to be similar to that of CVs except that there are no LMXRB with orbital periods between 1 and 2 hr, corresponding to the SU UMa and AM Her binaries. The evidence that the orbital periods of some LMXRB are evolving on a timescale of ≡10⁷yr is presented. Intriguingly, of the systems studied in detail two have increasing and two have decreasing orbital periods.