A DOUBLE-FREQUENCY DWARF NOVA OSCILLATION IN OY CAR

We have detected coherent oscillations ('dwarf nova oscillations') in Hubble Space Telescope spectra of the dwarf nova OY Car. The oscillati ons were seen towards the end of a superoutburst of OY Car. The oscill ations are extraordinary compared with the many other examples in the literature for two reasons. First, their amplitude is large, with a pe ak-to-peak variation of 8 to 20 per cent of the total flux over the ra nge 1100 to 2500 Angstrom. However, most remarkably we find that there are two components present simultaneously. Both have periods close to 18 s (equivalent to 4800 cycle d(-1)) but they are separated by 57.7 +/- 0.5 cycle d(-1). The lower frequency component of the pair has a s trong second harmonic while its companion, which has about twice its a mplitude, does not. The oscillation spectra appear hotter than the mea n spectrum and approximately follow the continuum distribution of a bl ackbody with a temperature in the range 30 000 to 50 000 K. We tentati vely suggest that the weaker non-sinusoidal component could represent the rotation of the white dwarf, although we have been unable to recov er any such signal in quiescent data.