Dereddened (V−K,J−K) colour–colour diagram for Henize S stars (open triangles) and M giants (open circles, from Whitelock et al. 2000) and a few C stars (crosses, from Whitelock et al. 2000), as compared to model colours (grid) of different effective temperatures and C/O ratios, for log g = 0, M = 1 M_⊙, [Fe/H] = 0 and for [s/Fe] = +0 dex (left) or [s/Fe] = +1 dex (right). The effective temperature ranges from 4000 to 2700 K, from bottom to top, as labelled in the figure; models with identical effective temperatures are connected by dashed lines. Different C/O ratios from 0.5 to 0.99 are represented by solid lines, the leftmost for C/O = 0.991, next to that C/O = 0.971, etc. The small error bars on the lower right corner of the [s/Fe] = 1 panel (in V−K and in J−K, barely seen) represent the variation of the colours at T_eff = 3000 K of 2 M_⊙ models with respect to 1 M_⊙ models, 2 M_⊙ models being slightly bluer (Δ(V−K) = 0.1 mag) than 1 M_⊙ models. The arrow marks the reddening vector for A_V = 0.6 mag.