HRD positions of N206-FS 128a (WC4, blue asterisk) and N206-FS 128b (O9, red square). The solid lines represent binary evolutionary tracks from Eldridge et al. (2008) for the respective components. The circles denote the best-fitting positions along the tracks after 6 Myr of evolution. For comparison, the dashed lines show how the components would evolve without mutual interaction. See text for more details.