Abstract
The recent proposal that most AM Her systems have essentially identical white dwarf masses (∼ 0.6−0.7 |$M_\odot$|) implies a direct relation between their surface field-strengths B1 and magnetic moments. We show that for |$B_1\gt B_\text {max}\simeq 4\times 10^7 G$|, magnetic braking via the white dwarf magnetic field may drive the binary evolution so rapidly as to cause the effective disappearance of these systems. They end as detached systems with low mass degenerate companions and orbital periods |$P\le 7$| hr. This may explain the existence of apparently isolated, rapidly rotating and highly magnetic white dwarfs. Very old systems may appear as non-magnetic white dwarfs with detached brown dwarf companions. We stress that the apparent lack of unpolarized but synchronously rotating CVs at short orbital periods is a serious problem unless most magnetic CVs have |$B_1\ge 10^7$|. This limit is in rough agreement with that predicted by field-burying for 0.6−0.7 |$M_\odot$| white dwarfs.
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