The apsidal motion in close eccentric binaries is a means to unveil the internal structure of stars. I make use of it to test the internal mixing processes in stars with the stellar evolution code GENEC.

The apsidal motion is the slow precession of an eccentric orbit with time. Its rate depends on the tidal interactions occurring between the stars through k2, a measure of the star’s inner density profile. The apsidal motion rate is commonly derived from the eclipses’ times of minima, made possible thanks to high precision TESS/Kepler observations. I propose an innovative approach: derive the apsidal motion rate from radial velocities obtained over a long timescale combined with light curves to get high-accuracy consistent physical and orbital parameters for the binaries. I highlight recent results concerning the two most massive binaries studied this way.

Confronted to observations of massive stars, standard non-rotating single star models usually predict stars with too low a density contrast; the well-known k2-discrepancy. I built bespoke GENEC stellar evolution models including tidally-enhanced/suppressed rotational mixing for the twin massive binary HD 152248. The models reveal the instabilities allowing to reproduce the stellar density profiles: advecto-diffusive models better reproduce k2 than magnetic models. A large overshooting is necessary to converge towards the observed k2, yet alone is not sufficient. While a change in metallicity or mass-loss rate has no significant impact on k2, a larger initial helium abundance allows to better reproduce the k2. Yet, a super-solar helium abundance is not observationally supported. These analyses highlight the need for a process in the stars that slows down the increase of their radius with time. It paves the way for the next generation of models.