Nucleosynthesis predictions for Tc and Zr abundances during successive TDU events, shown as circles along the model tracks. Measured abundances in Tc-rich S stars (filled squares) from Shetye et al. (2021), are colour-coded by C/O ratio. Blue crosses highlight the two Tc-rich MS stars (AA Cam and o¹ Ori) in their sample. The grey box highlights the expected locus of Tc-rich M stars, where the Tc range reflects the detection threshold for Tc-rich classification (see Fig. 1). The upper limit of the Zr range corresponds to the dashed black line in Fig. 2 with [s/Fe] = 0.2, which separates M from S stars.