Accretion and magnetic field structure in AM Herculis systems

Detailed spectroscopic studies of the magnetic white dwarfs: in the AM Herc ulis type systems have shown that in most systems the magnetic white dwarf has two dominant poles with field strengths that differ by a factor similar to1.5-2 indicating non dipolar field structures. In all but the highest fi eld system AR UMa, phase locking appears to be maintained through magneto-s tatic interactions between the magnetic field of the white dwarf and the dy namo generated magnetic held of the secondary star. (C) 2000 Elsevier Scien ce B.V. All rights reserved.