AE Aquarii: how cataclysmic variables descend from supersoft binaries

AE Aquarii (AE Aqr) is a propeller system. It has the shortest spin period among cataclysmic variables (CVs), and this is increasing on a 10(7) yr time-scale. Its ultraviolet spectrum shows very strong carbon depletion versus nitrogen, and its secondary mass indicates a star far from the zero-age main sequence. We show that these properties strongly suggest that AE Aqr has descended from a supersoft X-ray binary. We calculate the evolution of systems descending through this channel, and show that many of them end as AM CVn systems. The short spin-down time-scale of AE Aqr requires a high birth rate for such systems, implying that a substantial fraction of cataclysmic variables must have formed in this way. A simple estimate suggests that this fraction could be of the order of one-third of current CVs. We emphasize the importance of measurements of the C/N abundance ratio in CVs, particularly via the C IV 1550/N v 1238 ratio, in determining how large the observed fraction is.