a)V vs. V − I for stars towards Pismis 24 from HST/WFPC2 images. The full line marks the ZAMS, the dashed lines are isochrones for (from the right) 1, 3, and 10 Myr old PMS stars obtained from the evolutionary tracks of Palla & Stahler (1999). These have been reddened by A_V = 5.5 mag and scaled to 1.7 kpc. The loci of 1 and 0.4M_⊙ are labelled near the 1 Myr isochrone. Large open triangles are stars without a NIR counterpart, whereas small blue full triangles are stars with X-ray detection. b)K_s vs. H − K_s for the NIR infrared counterparts of the optical stars (red open squares: stars on the left of the optical ZAMS; full squares: stars on the right of the optical ZAMS; blue full triangles: stars on the right of the optical ZAMS also detected in X-ray emission). The full lines are the isochrones for (from the right) 1 and 3 Myr old PMS stars obtained from the evolutionary tracks of Palla & Stahler (1999), same reddening and distance as above. The loci of 1 and 0.4 M_⊙ are also labelled and the dashed lines are their reddening paths (for 1 Myr old stars, up to A_V = 10 mag, according to Rieke & Lebofsky 1985). c)J − H vs. H − K_s for the NIR infrared counterparts of the optical stars (same symbols as above, but stars on the left of the optical ZAMS are omitted). The full line is the unreddened MS, the dashed lines are reddening paths with crosses every A_V = 10 mag, following Rieke & Lebofsky (1985). Also shown as full lines, the reddening paths according to the extinction law derived by Straiz˘ys & Laugalys (2008). The dot-dashed line marks the locus of Classical T Tauri Stars (Meyer et al. 1997). A colour version of this figure is available in the on-line edition.