N/C ratio as a function of N/O. The same symbols and color coding as in Fig. 9 have been adopted. Stellar evolution models according to Brott et al. (2011a) for M_∗ = 20, 25, 40 and 60 M_⊙ are overplotted. Here again, the same line styles as in Fig. 9 were adopted; we also color-scoded each corresponding mass range for clarity. The upper left panel presents the complete sample, the three remaining panels display the three mass ranges discussed in the text.