Tidal synchronisation of the subdwarf B binary PG 0101+039

Aims. Tidally locked rotation is a frequently applied assumption that helps to measure masses of invisible compact companions in close binaries. The calculations of synchronisation times are affected by large uncertainties, in particular for stars with radiative envelopes, calling for observational constraints. We aim to verify tidally locked rotation for the binary PG 0101+039, a subdwarf B star + white dwarf binary from its tiny (0.025%) light variations measured with the MOST satellite (Randall et al. 2005). Methods. Binary parameters were derived from the mass function, apparent rotation and surface gravity of PG 0101+039 assuming a canonical mass of 0.47 M-circle dot and tidally locked rotation. The light curve was then synthesised and was found to match the observed amplitude well. Results. We verified that the light variations are due to ellipsoidal deformation and that tidal synchronisation is established for PG 0101+039. Conclusions. We conclude that this assumption should hold for all sdB binaries with orbital periods of less than half a day. Hence the masses can be derived from systems that are too faint to measure tiny light variations.