STROBOSCOPIC DOPPLER TOMOGRAPHY OF FO AQR

FO Aqr is a close binary star in which a magnetic white dwarf accretes from a cool companion. Light curves and spectra show variations on th e orbital frequency, the white dwarf's spin frequency and combinations of the two. It is not clear from observation or theory whether accret ion in FO Aqr is mediated by stream, a disc or a combination of both. We have taken 938 spectra of FO Aqr with the 3.9 m AAT. The high signa l-to-noise ratio of our spectra enable us to see variations in the lin es directly without recourse to phase folding. HeII 4686 shows an 'S'- wave on the orbital period but on top of this sharp features run acros s from red-to-blue and blue-to-red, the direction reversing with orbit al phase. We apply the method of Doppler tomography in an effort to un derstand these features. To cope with the variations that occur as the white dwarf rotates we compute Doppler images as a function of the or ientation of the white dwarf relative to the binary system. The proces s is analogous to the use of a stroboscope in freezing the motion of r otating objects. In animated form a spot of emission is seen in the re gion of the gas stream and secondary star. The spot changes brightness and position as the white dwarf rotates. The movement in position is consistent with an azimuthally extended structure extending 120 degree s around the white dwarf. It is this movement that corresponds to the sharp features which run across the 'S'-wave. We show that these featu res are the cause of variations in the ratio of blue to red-shifted em ission that were formerly interpreted as signs of an eclipse in FO Aqr . There is no evidence for a disc in FO Aqr from our data. Models of t he secondary star and gas stream go some way to explaining the variati ons in spot position and flux, but the observed structure appears more extended than the models can fit. We speculate that magnetospheric in teraction may help solve this problem.