Multicolour observations of rapid oscillations in WZ Sagittae

We present an analysis of high-speed multicolour photometry of the dwarf nova WZ Sagittae (WZ Sge). We have updated the orbital ephemeris and found that P lies within the limits − 4.53 × 10⁻¹³ s s⁻¹ and 1.53 × 10⁻¹³ s s⁻¹ (3σ limits). We have detected rapid coherent oscillations at three different epochs with periods of 28.95, 29.33 and 28.13 s, respectively. The detection of the oscillation with period 29.33 s is marginal. The amplitude and phase of the oscillations have been found to vary over time-scales of a few tens of minutes. The colours of the oscillations can be represented by a power-law spectrum (fv∝v^a) with α≃ 1.8, a blackbody spectrum of temperature T≃ 120 000 K or a hydrogen white dwarf spectrum with log g=8 and T≃100 000 K. The colours of the oscillation amplitudes are not consistent with those expected from a ZZ Ceti type white dwarf (with a temperature of 15 000 K). We conclude that the oscillations in WZ Sge do not arise from direct viewing of the surface of a pulsating white dwarf.