Left panel: colour–magnitude diagram for NGC 2451A. Symbols are as in Fig. 1, with several cluster members that are known binaries being distinguished by filled green circles, and the bracketing isochrones having values of log t = 7.3 (leftmost) and log t = 7.8 (rightmost). Right panel: theoretical HRD for NGC 2451A. Symbols are as in Fig. 1, with the known binaries again distinguished by green. Bracketing isochrones corresponding to log t = 7.6 (leftmost) and log t = 7.8 (rightmost) are shown. Evolution tracks for 1.4,1.7,2.0,2.5,3.0,4.0,5.0,6.0, and 7.0 M_⊙ stars are shown.