[Zr/Fe] vs [Nb/Fe] ratios for the sample stars (blue star symbols), as compared with intrinsic (triangles) and extrinsic (open squares) S stars from Shetye et al. (2018, 2019). The yellow-green curves are the nucleosynthesis predictions from STAREVOL for 3 M_⊙, [Fe/H] = −0.2 models. Each symbol along these curves represents a thermal pulse. The 2 M_⊙ models completely overlap the 3 M_⊙ ones, but they only reach up to [Zr/Fe] ∼ 1.3.