What apsidal motion reveals about the interior of massive binary stars

Apart from asteroseismology, the most efficient observational technique allowing to probe the internal structure of a star is the determination of the apsidal motion in close eccentric binary systems. This secular precession of the major axis of the bi-nary orbit depends on the tidal interactions between the two stars. The rate of this motion is directly related to the internal structure of the stars, in particular their inner density profile. Based on radial velocity and light curve measurements made over a long timescale, the rate of apsidal motion can be constrained, together with the fundamental parameters of the stars. Comparing the observationally determined parameters to theoretical models of stellar structure and evolution then constrains the internal structure of the stars. This powerful technique has been known for years, but has been seldom applied - we are reviewing its interest and reveal recent results.