Complexity in close binary evolution

The complex nature of close binary evolution is briefly reviewed. Population synthesis calculations of zero‐age cataclysmic variable stars (ZACVs) are presented using a common envelope (CE) efficiency parameter, α_CE, that is a function of the core mass of the primary. Specifically, we investigate the possibility that α_CE is different for CE evolution involving red giant branch (RGB) primaries than for asymptotic giant branch (AGB) primaries. We calculate a sequence of model orbital period distributions in present‐day ZACVs for 11 different combinations of efficiency parameters (α_RGB, α_AGB). We find that if CE evolution is much less efficient for RGB primaries than for AGB primaries (α_RGB ～ 0.1 or less), the number of CVs that form below 2 h is significantly decreased compared with standard constant α_CE models. © 2008 Wiley Periodicals, Inc. Complexity, 2008.