Includes abstract.Includes bibliographical references.Quasi-periodic super-orbital modulations are an established feature of several luminous X-ray binaries. The mechanisms that have been proposed to be responsible for such modulations include irradiation-driven warping/tilting and tidally-induced precession of the accretion disc, jet precession and the modulation of the mass-transfer rate due to an accretion disc instability or the effects of a third body