We discuss aspects of the observational data on cataclysmic binaries and note possible correlations between type of behaviour and binary period. A gap between 2 and 3 hr in binary periods is judged to be real. A simple numerical procedure for evolving Roche-lobe-filling stars is described, and applied to white dwarf-red dwarf binaries for various mass loss and angular momentum loss mechanisms, and initial conditions. The results, in which we ignore the short-time-scale behaviour of the systems, are classified into four modes of evolution: normal, nuclear evolution dominated, angular momentum loss dominated and hydrodynamical. The clustering below 2 hr is interpreted in terms of evolution following the hydrodynamical mode, and we suggest that both stars in such systems are of low mass. This may be the commonest type of cataclysmic binary. A possible explanation for the apparent clustering of classical novae to periods of 3-5 hr is given, and evolutionary schemes for cataclysmic binaries outlined. We suggest, following Ritter and Webbink, that the short-period systems ( 2 hr) arise mainly from late case B mass transfer in the original binary and the longer period systems mainly from case C. © 1980 Royal Astronomical Society. Provided by the NASA Astrophysics Data System.
