Geneva colours computed from synthetic spectra of S stars with C/O = 0.90 (left panels) and C/O = 0.99 (right panels). Models are computed for [Fe/H] = 0.0 and [s/Fe] = 1 dex. Top panel: U−B₁ and B₂−V₁ as a function of surface gravity. Model temperatures range from 3000 K (thick continuous line) to 4000 K (thin dotted lines), by steps of 200 K (corresponding to the 4 intermediate continuous lines). Bottom panel: (U−B₁, B₂−V₁) plane for different gravities ranging from log g = 0 (thick continuous line) to log g = 5 (thin dotted line), as labeled, and for temperatures ranging from 4000 K (smaller indices) to 2700 K. These figures illustrate the sensitivity of these colours to gravity, especially for low-gravity (log g < 3) warm (T_eff≥3400 K) stars.