[Zr/Fe] as a function of [Nb/Fe] for the intrinsic S stars (black filled triangles) and extrinsic S stars (open squares) from S18 compared with the nucleosynthesis predictions for different initial masses and metallicities. The bitrinsic S stars from S20 are plotted with a blue star symbol. The “ext” and “int” labels next to the model parameters refer to extrinsic (respectively intrinsic) model abundances.