Upper left panel: surface N/C ratios during the RSG phase normalized to the initial ratio in logarithm and in mass fraction as a function of the luminosity for various models. The red full dots are for observed values; stars labeled 2, 3, and 6 are taken from Origlia et al. (2013), α Ori from Lambert et al. (1984), and IRS 7 from Carr et al. (2000). Upper right panel: same as upper left panel, but for the N/O surface abundance ratio. Lower left panel: tracks during the RSG phase in the N/C versus N/O plane (surface values). The models are the same as in the upper left figure. Lower right panel: surface ¹²C/¹³C ratios for the same models as in the upper left figure. The dots are observed values for 6, α Sco (Hinkle et al. 1976), 3, 2, alpha Ori, and VY CMa (Matsuura et al. 2014).