  Regular observations by the All Sky Monitor aboard the Rossi X-ray Timing
Explorer satellite have yielded well-sampled light-curves with a time baseline
of over ten years. We find that up to eight of the sixteen brightest persistent
low mass X-ray binaries show significant, possible sinusoidal, variations with
periods of order ten years. We speculate on its possible origin and prevalence
in the population of low mass X-ray binaries and we find the presence of a
third object in the system, or long-period variability intrinsic to the donor
star, as being attractive origins for the X-ray flux modulation we detect. For
some of the objects in which we do not detect a signal, there is substantial
short-term variation which may hide modest modulation on long time-scales.
Decade time-scale modulations may thus be even more common.
