A NEW EVOLUTIONARY MODEL FOR AM HERCULIS BINARIES

Observations of AM Herculis binaries recently discovered by ROSAT and other surveys have shown that the AM Herculis binaries do not exhibit a pronounced 2-3 h period gap, which is difficult to explain in terms of the canonical magnetic braking model for the orbital evolution of t hese binaries. We present a new evolutionary scenario for AM Herculis binaries based on the hypothesis that magnetic braking by the stellar wind of the M-star secondary ceases to be an effective mechanism for t he loss of orbital angular momentum when the rotation of the white dwa rf becomes synchronized with the orbital motion. This scenario not onl y explains the high proportion of AM Herculis binaries in the period g ap, but also provides a natural explanation for the low mass-transfer rate in these binaries compared to other cataclysmic variables and for the existence of an apparent observed upper limit to the surface magn etic fields of the white dwarfs that is significantly less than the ob served maximum magnetic field strength in isolated magnetic white dwar fs.