Band strength indices, B_TiO and B_ZrO, for Tc-rich and Tc-poor M stars from this study, and for Tc-rich (intrinsic) and Tc-poor (extrinsic) S stars from Shetye et al. (2021). These are represented by filled or open squares and triangles, as indicated in the legend. Corresponding spectral indices are also computed from synthetic spectra based on model atmospheres with varying stellar parameters, including effective temperature (colour-coded according to the colour bar), metallicity, C/O ratio, s-process enrichment [s/Fe], alpha-elements enrichment [α/Fe], and microturbulence, χ_t. Band strengths derived from synthetic spectra are shown as crosses, with models of [Fe/H] = 0 connected by dashed lines and those with [Fe/H] = –1 connected by solid lines.