A boundary layer origin for dwarf nova oscillations

Dwarf nova oscillations (DNOs) have been observed in a number of cataclysmi c variables. I propose that these oscillations could be produced by a non-a xisymmetric bulge at the transition between the optically thick disc and th e optically thin boundary layer region. This would naturally explain the ob served oscillation periods and the dependence of the oscillation amplitude on photon energy. The transition radius moves inward and outward with chang ing mass accretion rate, which explains the correlation between period and flux, and the time-scale for period variations. The underlying cause of the non-axisymmetry that produces the oscillations is not known, so it is not possible to predict the oscillation amplitude from first principles.