Measured B_TiO and B_ZrO band indices from observed spectra of different stellar types (as labelled) compared to band indices computed from synthetic spectra for the same grid of temperatures (models with identical – as indicated – effective temperatures joined by dashed black lines) and C/O ratios (connected by solid coloured lines, for the same values as those displayed in Fig. 5, the largest C/O value corresponding to the most vertical solid line). The four panels correspond to various combinations of metallicity and [s/Fe] ratio, as labelled. Field stars (radial velocity standards and flux calibrators without molecular bands) are clumping around (B_TiO, B_ZrO) = (0, 0), as expected. The small error bars on the lower right corner of the [Fe/H] = 0, [s/Fe] = 1 panel represent the largest variations (computed at 3000 K, 3400 K and 3800 K) of the indices of 2 M_⊙ models with respect to 1 M_⊙ models, 1 M_⊙ models having slightly stronger lines (and larger indices) than 2 M_⊙ models.