CHANGES OF ACCRETION SPOT LONGITUDE IN ECLIPSING AM HERCULIS BINARIES

Photometry of the eclipsing AM Herculis systems WW Hor and DP Leo obtained during 1992 shows that the longitude of the accretion spot in both systems is significantly different from that measured several years ago. In WW Hor, the spot longitude has changed from psi = 11.degrees2 +/- 1.degrees8 in 1987 November to - 3.degrees6 +/- 2.degrees5 in 1992 July. In DP Leo, the change is from psi = - 14-degrees +/- 4-degrees in 1984 March to + 4-degrees +/- 3-degrees in 1992 March. The changes are consistent with models which assume that phase locking is maintained by the magnetostatic interaction between the intrinsic field distributions of the component stars. These models predict that the magnetic axis of die white dwarf should oscillate about its equilibrium position on a time-scale of approximately 50 yr. We also present a high time resolution eclipse light curve of DP Leo which resolves the eclipse of the white dwarf and the accretion spot.