Evolution of k₂ and . Left panel: evolution as a function of stellar age of the internal structure constant (to which the empirical correction Eq. (22) has been applied) for Clés models with initial mass of 19.0 M_⊙ and ξ = 1 (k_2, 1, green) and initial mass of 7.7 M_⊙ and ξ = 0.1 (k_2, 2, blue), both models have α_ov = 0.30 and no turbulent diffusion. The weighted-average mean of the k₂-values, computed using Eq. (19), is also depicted (, purple). Right panel: evolution as a function of stellar age of the total apsidal motion rate (purple) and of the contributions of the two stars (green and blue). The observational value of the corresponding parameter and its error bars are represented by the solid red line and the dashed red horizontal lines, respectively.