  Long-term ("superorbital") periods or modulations have been detected in a
wide variety of both low and high-mass X-ray binaries at X-ray and optical
wavelengths. A variety of mechanisms have been proposed to account for the
variability properties, such as precessing and/or warped accretion discs,
amongst others. The All Sky Monitor on board the Rossi X-ray Timing Explorer
provides the most extensive (~15 years) and sensitive X-ray archive for
studying such behaviour. It is also clear that such variations can be
intermittent and/or a function of X-ray spectral state. Consequently, we use a
time-dependent Dynamic Power Spectrum method to examine how these modulations
vary with time in 25 X-ray binaries for which superorbital periodicities have
been previously reported. Our aim is to characterize these periodicities in a
completely systematic way. Some (such as Her X-1 and LMC X-4) are remarkably
stable, but others show a range of properties, from even longer variability
time-scales to quite chaotic behaviour.
