Dwarf nova oscillations and quasi-periodic oscillations in cataclysmic variables - I. Observations of VW Hyi

From archived and recent high-speed photometry of VW Hyi we find dwarf nova oscillations (DNOs) occasionally present throughout outburst, evolving from a 14.06-s period at maximum to >40s near the end of outburst. A relatively slow increase of period is followed by a rapid increase and a subsequent decrease. Quasi-periodic oscillations (QPOs) are seen at periods of hundreds of seconds. For the first time, the evolution of a QPO period is seen, increasing steadily during the final decline of an outburst. The occasional presence of two DNOs, separated in frequency by the QPO frequency, suggests reprocessing of the rotating DNO beam by a `wall' rotating progradely in the disc at the QPO period.