Spin evolution and magnetic fields in cataclysmic variables

The present study examines a number of intermediate polars which are observed to have spin periods of very close to 10 percent of the orbital period. It is shown that, if they spin in equilibrium, the white dwarfs in these systems accrete essentially the specific angular momentum of the secondary star, averaged over time scales of about 10,000 yr. It is shown that some kind of disk, e.g., a nonaccretion disk, may be present in IPs if most of the matter is not accreted through it. It is proposed that nonsynchronous systems with magnetic moments not less than 10 to the 34th G cu cm are unlikely to be able to accrete gas of low enough density to produce hard X-rays. The progenitors of the AM Herculis systems are thus effectively unobservable; in particular, they cannot be identified with the observed IPs. It is concluded that the observed IPs have magnetic moments that are significantly lower than the AM Herculis systems, and the secular evolution of magnetic cataclysmic variables is reconsidered.