C/O vs. [Fe/H] of FGK dwarf stars from Delgado Mena et al. (2021). Also plotted is the evolution of C/O of three COLIBRI TP-AGB models from Marigo et al. (2013) with masses of 1.5, 1.8, and 2.0 M_⊙. All models are calculated at [Fe/H]= − 0.03, but the models with 1.5 and 2.0 M_⊙ are shifted in [Fe/H] for clarity. Each symbol on the TP-AGB tracks marks a 3DUP episode. The horizontal dashed line marks C/O = 1, above which the 1.8 and 2.0 M_⊙ models become carbon-rich.