The period gap and the AM Herculis type systems

We argue that the period distribution of AM Herculis binaries in the enlarged sample incorporating results from the recent ROSAT X-ray survey differs significantly from that of other cataclysmic variables. In particular, there is no evidence for a pronounced period gap at 2—3 hr and the significance of the period spike at about 114 min is brought into question. We present an alternative evolutionary scenario for the AM Herculis binaries based on the hypothesis that magnetic braking by the stellar wind of the M star secondary either ceases or is severely curtailed when the rotation of the magnetic white dwarf becomes synchronised with the orbital motion, The orbital evolution of the AM Herculis binaries is thereafter driven mainly by angular momentum loss due to gravitational radiation. This scenario not only explains the higher proportion of AM Herculis binaries in the period gap when compared with other cataclysmic variables but also provides a natural explanation for the low mass transfer rates in these binaries and for the existence of an apparent upper limit for the surface magnetic fields of their white dwarfs.