Relation ϵ_C vs. ϵ_N in the light of evolutionary tracks. The black and red lines display the corresponding Geneva and Bonn tracks with , respectively, where the solid lines refer to the 60 M_⊙, the dashed lines to the 40 M_⊙, and the dashed-dotted lines to the 20 M_⊙ tracks. The dotted lines represent the solar C and N abundances, and their intersection the position of the sun in this diagram. Symbols as in Fig. 1. Error bars include typical uncertainties in the stellar parameters.