Location of single and binary evolutionary models from Jin et al. (2024, and in prep.) in the N vs. He diagram. The extended pink line covers the evolutionary tracks of the SSEM described in Appendix C. Mass gainers predictions are depicted immediately before mass transfer (MT; cyan dots), and after mass transfer at a time when thermal equilibrium is restored (magenta dots). Left and right panels depict MESA calculations assuming an inefficient or conservative mass transfer, respectively. The dotted-blue line shows abundance ratios under the assumption of complete CNO-equilibrium at any given He abundance. The dotted-green line has the same meaning but assuming a mixture between matter with the initial chemical composition and matter consisting of pure He and the CNO-equilibrium value of N (with a mass fraction indicated by along the red dots). Both dotted lines frame the allowed space for evolutionary models. The dotted-black line refers to a mixture of matter with the initial abundances with matter with a He mass fraction of Y = 0.63, corresponding to the average in the He-enriched part of the donor envelopes, and the CNO-equilibrium value of N.