  Most transiting planets orbit very close to their parent star, causing strong
tidal forces between the two bodies. Tidal interaction can modify the dynamics
of the system through orbital alignment, circularisation, synchronisation, and
orbital decay by exchange of angular moment. Evidence for tidal circularisation
in close-in giant planet is well-known. Here we review the evidence for excess
rotation of the parent stars due to the pull of tidal forces towards spin-orbit
synchronisation. We find suggestive empirical evidence for such a process in
the present sample of transiting planetary systems. The corresponding angular
momentum exchange would imply that some planets have spiralled towards their
star by substantial amounts since the dissipation of the protoplanetary disc.
We suggest that this could quantitatively account for the observed mass-period
relation of close-in gas giants. We discuss how this scenario can be further
tested and point out some consequences for theoretical studies of tidal
interactions and for the detection and confirmation of transiting planets from
radial-velocity and photometric surveys.
