Super-orbital periods in X-ray binaries

Super-orbital periods have been reported from more than 30 X-ray binaries, but no consistent picture has emerged as to the underlying mechanism behind these periods. Only four of the sources have exhibited stable super-orbital periods over intervals of several years, and three of these are classed as high mass X-ray binaries. The remaining sources are split approximately equally between low mass and high mass binaries. We have searched for correlations with known properties of the binary systems for all the sources, viz. compact object spin periods, orbital periods, accretion disc precession, and companion star properties. We present the results of our investigations.