Superhump

Superhumps were first seen in SU Ursae Majoris (SU UMa) stars, a subclass of dwarf novae, at times when the binary system underwent a superoutburst, which is an unusually strong outburst (increase in brightness) caused by an increased accretion rate.

[2] The accretion disk is elongated by the tidal force of the donor star.

The elliptical disk precesses around the white dwarf accretor over a time interval much longer than the orbital period, the beat period, causing a slight change in the orientation of the disk over each orbit.

[3] Superhumps in cataclysmic variable stars are the result of viscous dissipation by periodic deformations of the disk.

These deformations are caused by the presence of a 3:1 resonance between the orbital periods of the accretion disk and the donor star.