Comparison between the observed value of (solid horizontal line) with its errors (dashed horizontal lines) and the values evaluated from stellar structure models as a function of stellar age. The solid curve yields the results obtained by interpolating the models of Claret & Giménez (1992). The filled squares correspond to the results obtained through the resolution of the Clairaut-Radau equation applied to the CLES models for a 20 M_⊙ primary and a 15 M_⊙ secondary. The open squares yield the results for CLES models assuming an overshooting with α_ov = 0.2. The filled diamonds stand for the CLES models with a slightly higher metallicity.