Left: evolutionary tracks for stellar models with masses equal to 12.0 and 10.6 M_⊙ and different values of V_rot (see the labels). The OP opacities and initial composition of X = 0.7374 and Z = 0.0134 were adopted. Every tenth model for the non-rotating case is indicated with a dot, starting from model #10 (labeled). Three coeval pairs of models are connected with dashed lines and labeled with model number in the 12.0-M_⊙ evolutionary track; see text for explanation. Right: age (top) and central hydrogen mass abundance, X_c, (bottom) plotted as a function of model number. The relation for 12.0 M_⊙ is plotted with a continuous line, and for 10.6 M_⊙, with a dashed line.