The ([Zr/Fe], [Nb/Fe]) plane. Different kinds of stars are represented by different symbols as labelled in the lower right corner. Abundances for the barium stars are from Karinkuzhi et al. (2018). The error bars on [Zr/Fe] and [Nb/Fe] represent the total uncertainty on the abundances. The red and blue line represent the surface abundance predictions from the models. Each thermal pulse is indicated by a symbol: circles correspond to surface abundances of extrinsic stars, i.e. after the full decay of ⁹³ Zr into Nb, and squares to intrinsic S stars still on the TP-AGB.