HR diagram for the selected sample of EBs (see text). For each system we plotted the models for the labelled chemical composition and mass, corresponding to the primary (blue line) and the secondary star (red line). Models have been computed by adopting three different values for the mixing length parameter, namely α_PMS = 1.0 (solid line), 1.20 (dashed line), and 1.68 (dot-dashed line). Observational data are shown (filled circles), together with their uncertainty.