THE INITIAL PERIODS OF CATACLYSMIC VARIABLES AND THE MAXIMUM FIELD-STRENGTH IN AM HERCULIS SYSTEMS

We show that the majority (> 67 per cent) of cataclysmic variables (CV s) emerge from common-envelope (CE) evolution with fully convective se condaries, so that they start mass transfer near or below the CV perio d gap. Moreover, more than 80 per cent of pre-CVs emerge from CE evolu tion with periods less than or similar to 8 h, short enough to evolve into contact and become CVs under gravitational radiation alone within the lifetime of the Galaxy, without the help of magnetic stellar wind braking. It has recently been argued that the strong primary magnetic field might suppress magnetic stellar wind braking in most AM Herculi s binaries, and that the resulting evolution under gravitational radia tion would imply an upper limit to the white dwarf field for these sys tems. However, our results show that at least 80 per cent of AM Her sy stems form with periods too short for any such limit to apply to them. We conclude that such arguments do not explain any systematic feature of the AM Her field distribution.