Changes of accretion spot longitude in eclipsing AM Herculis binaries

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Abstract
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 ψ = 11°.2 ± 1°.8 in 1987 November to –3°.6 ±2°.5 in 1992 July. In DP Leo, the change is from ψ = – 14° ± 4° in 1984 March to +4° ± 3° 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 the white dwarf should oscillate about its equilibrium position on a time-scale of ~ 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.

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