[F/⟨s⟩] vs. [⟨s ⟩/Fe] in the sample stars with [Fe/H] > −0.5. As in Fig. 1, filled symbols correspond to the stars analysed here while open symbols are stars previously studied (see text). Colour code: black, normal C-stars; green, SC-stars; magenta, CH/Ba stars. Lines are theoretical predictions for a non-rotating 2.0 M_⊙ TP-AGB star at different metallicities (quoted in the figure), according to Cristallo et al. (2015). The continuous part of the lines indicates the theoretical C-rich phase while the dashed part shows the O-rich phase. We note that at these metallicities, theoretically, stars become C-rich during the AGB phase after several TPs. The middle continuous line corresponds to the model with Z = Z_⊙ but the label has been omitted for clarity.