Some aspects of the observational data on cataclysmic binaries are discussed and some possible correlations between type of behaviour and binary period are noted. A gap between 2 and 3 hours in the histogram of binary periods is estimated to be real. A numerical procedure for following the evolution of Roche-lobe-fiUing stars using simplified equations is described. This procedure Is applied to white/red dwarf binaries for a variety of initial conditions, and of mass loss and angular momentum loss mechanisms. The results of these calculations, in which we ignore the short timescale behaviour of the systems, are classified into four modes of evolution: normal, nuclear evolution dominated, angular momentum loss dominated and hydrodynamical. The results are discussed in connection with cataclysmic binaries. The clustering in period below 2 hours is Interpreted in terms of evolution following the hydrodynamical mode, and it is suggested that such systems contain low mass white dwarfs as well as low mass secondaries. These may be the most common type of cataclysmic binary. A possible explanation of the clustering of classical novae systems to binary periods of 3 to 5 hours is mentioned, and evolutionary scenarios for cataclysmic binaries are outlined. We suggest, following Ritter and Webbink, that the short period systems (≲ 2 hrs) arise mainly from late Case B mass transfer in the original binary (original primary mass 1.5 to 3M⊙) and the longer period systems arise mainly from Case C mass transfer.

Full text to be published in Monthly Notices of the Royal Astronomical Society.