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We review X-ray flux modulation from X-ray binaries on time scalescorresponding to the orbital period and those at longer time scales (so calledsuperorbital). Those modulations provide a powerful tool to constrain geometryof the accretion flow. The most common cause of the superorbital variabilityappears to be precession. We then discuss two specific examples of discoveriesof a coupling between the two types of variability and their physicalinterpretation. One is Cyg X-1, a black-hole binary with a high-mass companion,in which case we find the presence of an accretion bulge formed by collision ofthe stellar wind with the outer edge of the precessing accretion disc. Theother is 4U 1820-303, a neutron star accreting from a low-mass white dwarf, inwhich case we interpret the superorbital variability as accretion ratemodulation induced by interactions in a triple stellar system. Then, thevarying accretion rate leads to changes of the size of the accretion bulge inthat system, obscuring the centrally-emitted X-rays.

Orbital and superorbital variability and their coupling in X-ray binaries