Top panel: WISE F_W4/F_W1 against 2MASS J/K_s flux ratios for Algols from the catalogue of Budding et al. (2004), and for W Ser from the list of Gudel & Elias (1996). The symbiotic Algol SS Lep and a few Be and B[e] systems have been considered as well (as discussed in Sect. 4.1). The black filled triangles (linked by the dashed line) are black bodies of given temperatures (labels in Kelvin). The red filled squares represent the simulations (STAR: input spectrum). For clarity, only extreme models are labelled, and we refer to Table 5 for unlabelled models. The filled circles correspond to Algols with a S/N > 3.2. The size of the circle is proportional to the S/N for band W4 (except for SS Lep which has a S/N of 962 and for which we limit the size of the symbol), and the symbol colour relates to the χ² for the same band. Bottom panel: same as top panel but for F_W3/F_W1 with S/N and χ² relative to W3.