Average carbon-to-oxygen abundance ratio as a function of [O/H] in the stellar population of the prototype massive elliptical galaxy. The thick solid lines show the evolution of the mass-averaged abundances of the stellar population. The starlets mark the ending points of the theoretical tracks. Different shades of purple refer to different nucleosynthesis prescriptions, corresponding to different initial rotational velocities of the core-collapse SN progenitors (see text). The big grey star indicates the values of the ratios estimated from absorption line indices of local galaxies (Johansson et al. 2012).