Variations with time of the surface mass fraction of ¹²C (top-left panel), ¹⁴N (top right), lithium (bottom left), and the ¹²C/¹³C ratio (bottom right) of the same models shown in Fig. 4 (except for the 6 M_⊙ model, omitted here for the sake of readibility). The black dots refer to a 4 M_⊙ star of solar metallicity, published in Ventura et al. (2018), while the other symbols are the same as in Fig. 4.