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 (CVs) emerge from common-envelope (CE) evolution with fully convective secondaries, so that they start mass transfer near or below the CV period gap. Moreover, more than 80 per cent of pre-CVs emerge from CE evolution 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 Herculis binaries, and that the resulting evolution under gravitational radiation would imply an upper limit to the white dwarf field for these systems. However, our results show that at least 80 per cent of AM Her systems 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.