Left: Colour–magnitude diagram of NGC 3603, constructed using synthetic photometry (open black circles). Filled grey circles represent the WEBDA catalogue magnitudes (Sagar et al. 2001), shown for comparison. Isochrones of Z = 0.004, log t = 6.0 (1 Myr, cyan) and 7.0 (10 Myr, blue) are plotted with solid lines, those of the same ages at Z = 0.008 with dashed lines. Non-fitting isochrones of 20 Myr are also plotted (red). The position of the main-sequence turn-off point (MSTO) is indicated with a yellow circle. Right: two-colour diagram of NGC 3603, also constructed using synthetic photometry (open black circles). The solid black line indicates a linear regression, the dashed red line the normal interstellar extinction law. Following Sagar et al. (2001), colour excess ratios of E(V−R) /E(B−V) = 0.65 (Alcalá & Arellano Ferro 1988) and E(V−I) /E(B−V) = 1.25 (Dean et al. 1978) are used for the normal extinction law. The directions of the normal reddening vectors are shown with arrows in both diagrams.