Ar. King et al., THE INITIAL PERIODS OF CATACLYSMIC VARIABLES AND THE MAXIMUM FIELD-STRENGTH IN AM HERCULIS SYSTEMS, Monthly Notices of the Royal Astronomical Society, 269(4), 1994, pp. 907-912
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.