Secular evolution of magnetic cataclysmic variables

We show that the observed period distribution of magnetic cataclysmic variables can be easily understood on the basis of current theories of the secular evolution of these systems, and the formation of the period gap in particular, provided that the white dwarfs have magnetic moments µ either in the range $$10^{33}\lesssim\mu\lesssim10^{34}$$ G cm³, or µ<10³⁰ G cm³, roughly as observed for isolated white dwarfs. The absence of intermediate polars with orbital periods <2 hr rules out the possibility that their fields are systematically weaker than those of the AM Her systems, unless magnetic binaries never evolve across the period gap, and the field distribution is as above for orbital periods <2 hr. We show how the spin history of EX Hya fits with this picture, and discuss other estimates giving low values of µ for the intermediate polars.