K_s vs. H − K_s for the NIR counterparts of the YSOs towards Pismis 24 identified according to four different indicators, as explained in the text. Open symbols always refer to sources in the northern field, whereas full symbols refer to sources in the southern field. a) YSOs identified on the basis of their IRAC colours (triangles: Class I; squares: Class II); b) YSOs identified on the basis of their combined JH(HK_s) [4.5] colours; c) YSOs identified on the basis of their JHK_s colours; d) NIR counterparts of X-ray sources. The thick solid vertical line marks the main sequence locus (using absolute magnitudes from Allen 1976 and colours from Koornneef 1983) for a distance modulus of 11.15 mag (d = 1.7 kpc). The arrow in a) shows a reddening of A_V = 20 mag according to Rieke & Lebofsky (1985). Spectral types are labelled on the ZAMS. A colour version of this figure is available in the on-line edition.