A SURVEY FOR QPOS IN AM HERCULIS STARS AND A DETAILED STUDY OF THE QPOS IN AN URSAE MAJORIS

We have searched for quasi-periodic oscillations with frequencies between 0.1 and 2.4 Hz in the light curves of nine AM Her stars using simultaneous multi-colour Photometry. Oscillations were not seen in BY Cam (= H 0538 + 608), EF Eri, AM Her, DP Leo (= E 1114 + 182), ST LMi (= CW 1103 + 254), MR Ser (= PG 1550 + 191) or QQ Vul (= E 2003 + 225), but were detected in AN UMa and VV Pup. We were unable to find any systematic difference between those systems with and those without quasi-periodic oscillations. When averaged over the orbital period, the amplitude of the oscillations in AN UMa is largest in the B bandpass (approximately 3.5 per cent). The colour of the oscillations varies systematically with orbital phase, however, and the amplitude is at least as high in the U band as in the B band during the main hump in the light curve. The average coherence time as measured with autocorrelation techniques is 37 +/- 2 s for strong oscillations in the B band. We show that the accretion column is divided into 2-45 regions, each of which is oscillating simultaneously but independently. Our observations are consistent with the shock-oscillation model.