Gravitational wave emission by cataclysmic variables: numerical models of semi-detached binaries

Gravitational wave emission is considered to be the driving force for the e volution of short-period cataclysmic binary stars, making them a potential test for the validity of general relativity. In spite of continuous refinem ents of the physical description, a 10 per cent mismatch exists between the theoretical minimum period (P-turn similar or equal to 70 min) and the sho rt-period cut-off (P-min similar or equal to 80 min) observed in the period distribution for cataclysmic variable binaries. A possible explanation for this mismatch was associated with the use of the Roche model. We present h ere a systematic comparison between self-consistent, numerically constructe d sequences of hydrostatic models of binary stars and Roche models of semid etached binaries. On the basis of our approach, we also derive a value for the minimum period of cataclysmic variable binaries. The results obtained b y the comparison indicate that the Roche model is indeed very good, with de viations from the numerical solution which are of a few per cent at most. O ur results therefore suggest that additional sources of angular momentum lo ss or alternative explanations need to be considered in order to justify th e mismatch.