IRAC colour–colour diagrams. Top panel, [3.6] – [4.5] vs. [5.8] – [8.0] for sources detected in all four bands and with photometric errors <0.3 mag. Bottom panel, [3.6] – [4.5] vs. [4.5] – [5.8] for the same sources as above, plus those only detected in the three shorter wavelength bands (with photometric errors <0.3 mag). The two samples have been cleaned of contaminants as explained in the text. Following the prescriptions of Gutermuth et al. (2009) for their identification, Class I sources are in red and Class II sources in blue. Stars are marked as black dots. A reddening corresponding to A_V = 20 mag (according to Indebetouw et al. 2005) is marked by an arrow. In the top panel, the regions occupied by Stage I and Stage II sources (Robitaille et al. 2006) are also outlined.