\journalname
Astronomy&AstrophysicsReview
The other stellar parameter of importance in understanding tidal evolution in binary systems is the level of synchronisation of the component stars. Fig. \ref{vsini} shows the relation between the observed values of \(v\sin i\) and those expected from synchronisation with the orbital period. Some deviating cases are found, taking observational uncertainties into account, mainly in the sense of the observed rotations being faster than synchronous, but some cases of sub-synchronous rotation are also seen. Only the high quality of our data allows to identify these non-synchronous cases with confidence.
In eccentric systems, tidal forces vary over the orbital cycle, being strongest at periastron. One thus expects the stars to rotate at a rate intermediate between the orbital angular velocity at periastron and that expected for similar single stars. The rotation period of single stars is generally shorter than the typical orbital period of the systems in Table \ref{tableMR} for early-type stars with radiative envelopes, longer for late-type stars with convective envelopes. The speed with which the stars are spun up or down to their final rotational velocity will, of course, depend on the strength of the tidal forces in each system, i.e., primarily on the relative radii of the stars.
Calculations of the average effect of the tidal forces over an eccentric orbit lead to a prediction of the final net rotation of the components – the concept of pseudo-synchronisation as defined by \cite{1981A+A....99..126H}. Taking this as the best average prediction for the observed rotation rates, Fig. \ref{vps} shows the level of pseudo-synchronisation achieved by the stars as a function of relative radius (pre-main-sequence stars excluded). For the sake of clarity, stars with convective and radiative envelopes are shown in separate panels, and circular and eccentric orbits by different symbols.