HRD of the synthesized population of HeS stars. The gray scale shows the number of HeS stars per Δ(log(T_eff)) × Δ(log(L/L_⊙)) = 0.05 × 0.1 pixels. Upper panel: symbols are color-coded according to the HeS stars’ masses (scale to the right). Lower panel: symbols are color-coded according to the initial binary periods. Circles mark the positions of the computed models, like in Figs. 3 and 4.