(K− [12] ,K− [25]) (top) and (K− [12] , [25] − [12]) (bottom) colour–colour diagram of Henize S stars. Extrinsic S stars are represented with squares, intrinsic S stars with triangles, and S stars for which no assignment is available (Van Eck & Jorissen 2000a) with circles. The K magnitude is from 2MASS. Only the good-quality (flag = 3) IRAS fluxes have been used. The insets show a wider scale, where 2 outliers (Hen 4-155 and Hen 4-171) are included. The dashed line on the upper panel represents blackbody colours, with arrows corresponding to temperatures ranging between 4000 and 1500 K, as labelled.