Colour–colour diagram of our sample. Class I, II, and III YSOs are plotted as green, red, and blue crosses, respectively. Triangles are lower limits. Class I YSOs which show HI in emission are surrounded by a black circle. The main sequence (MS, Luhman et al. 2010) and CTT locus (Meyer et al. 1997) are shown in black and purple, respectively. The green arrow in the left-top corner defines the reddening direction, following the Cardelli et al. (1989) extinction law. Its length corresponds to A_V = 5 mag.