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