A double-frequency dwarf nova oscillation

We have detected coherent oscillations (``dwarf nova oscillations'') in Hubble Space Telescope spectra of the dwarf nova OY Car. The oscillations were seen towards the end of a superoutburst of OY Car. The oscillations are extraordinary compared to the many other examples in the literature for two reasons. First, their amplitude is large, with a peak-to-peak variation of 8 to 20% of the total flux over the range 1100 to 2500A. However, most remarkably we find that there are two components present simultaneously. Both have periods close to 18sec (equivalent to 4800 cycles/day) but they are separated by 57.7+/-0.5 cycles/day. The lower frequency component of the pair has a strong second harmonic while its companion, which has about twice its amplitude, does not. The oscillation spectra appear hotter than the mean spectrum and approximately follow the continuum distribution of a black-body with a temperature in the range 30,000 to 50,000K. We tentatively suggest that the weaker non-sinusoidal component could represent the rotation of the white dwarf, although we have been unable to recover any such signal in quiescent data.