Distribution of the carbon abundance of our sample stars with respect to their [s/Fe] index. Top panel: measured [C/Fe] abundances as a function of [s/Fe] for our large sample of Tc-rich S stars (light green, purple and orange squares) where the size of the symbol increases with decreasing metallicity, considering three metallicity bins: [Fe/H] in [0.0; −0.2]: light green, [−0.2; −0.4]: purple, below −0.4: orange. Tc-rich carbon stars from Abia et al. (2002) are shown as green squares. Bottom panel: derived C/O ratio versus [s/Fe] for the same stars. Predictions for a 2 M_⊙ STAREVOL model at metallicity [Fe/H] = 0.0 (light green), −0.3 (violet) and −0.5 (orange) are overplotted. The empty circles along the tracks indicate the successive TDUs, while the three filled circles mark the first TDU, which led to C/O > 1 in each model.