HR diagrams for six O-type main-sequence massive stars (Pis 24-2, 13, 16, 3, 15, 10, filled circles), and two B-type main-sequence stars (Pis 24-12, 19, open circles) in Pismis 24. The massive stars are marked with numbers (see Table 2). The dashed lines represent the zero-age main sequence locus. The thin solid lines show the isochrones of 2, 3, 4, 5, 6, 8, and 10 Myr. The thick solid lines present the best-fitting isochrones of 1, 2.2, and 2.7 Myr in panels a)– c), respectively. The evolutionary tracks are from Schaller et al. (1992). The fitting gives a distance of 1.7  ±  0.2 kpc. d) HR diagram for massive stars with an assumed distance of 2.56 kpc. There is no single isochrone that fits all six O-type main sequence stars.