  This paper reviews the basic equations used in the study of the tidal
variations of the rotational and orbital elements of a system formed by one
star and one close-in planet as given by the creep tide theory and Darwin's
constant time lag (CTL) theory. At the end, it reviews and discusses the
determinations of the relaxation factors (and time lags) in the case of host
stars and hot Jupiters based on actual observations of orbital decay, stellar
rotation and age, etc. It also includes a recollection of the basic facts
concerning the variations of the rotation of host stars due to the leakage of
angular momentum associated with stellar winds.
