ROSAT SKY SURVEY OBSERVATIONS OF THE ECLIPSING BINARY V471 TAURI

We present ROSAT observations of the DA white dwarf + K2V binary system V471 Tauri, obtained during the sky survey phase of the mission. The almost-equal-to 5 d of data span a number of the 0.52-d binary cycles, allowing us to study the eclipse of the white dwarf by the K star. In addition, a lower amplitude shorter time-scale variability is seen in both the soft X-ray and EUV bands. We associate this with the white dwarf pulsations previously discovered by EXOSAT and also observed at optical wavelengths. Using an accurate ephemeris for the optical pulsations, it is possible to reconstruct the optical pulse shape for the epoch of the ROSAT observations. Comparison of this with the EUV data, folded by the main 554.63-s optical period, reveals that the minimum in the EUV light curve coincides with maximum in the optical. This is the first direct comparison of the phases of the optical and EUV pulses. It confirms the prediction made by an earlier indirect comparison and shows conclusively that the V471 Tau oscillations cannot arise from non-radial g-mode pulsations in the white dwarf. They are most likely caused by rotation of the white dwarf with accretion-darkened magnetic poles. From the EUV and optical pulse shapes we have studied the accretion geometry and estimated that the rate of accretion on to the white dwarf is almost-equal-to (4-11) x 10(-13) M. yr-1.