1999A&A...343..132C

Four AM Herculis systems are known to have slightly asynchronous white dwarf primary stars, and several others show features which could be consistent with some form of asynchronism. We address the problems of the attainment and maintenance of corotation, and show why it is possible that some systems may have asynchronous motions, while others can be locked. Differences in system parameters such as the white dwarf mass and magnetic field are related to the existence and attainment of locked states. Oscillations about a locked state are also possible with typical periods of a few decades. Given the constraints necessary to attain a stable synchronous state, we note that there are probably more asynchronous systems than those known. Theory suggests that there may be systems with degrees of asynchronism two orders of magnitude lower than those presently observed. We emphasise the need for observations of AM Her binaries over longer time bases in order to search for such systems.