Dynamical masses as a function of spectral type for main-sequence O stars. Symbols in black denote eclipsing systems as described in Sect. 5.1.1. Symbols in red denote the lower dynamical limit for the non-eclipsing secondary in WR21a and the primary and secondary components of HD 150136, with the dotted lines connecting to spectroscopic mass estimates for the latter two stars. Error bars for these two objects reflect uncertainties in their spectral type. Small offsets have been applied to individual stars of spectral types O5.5, O6, O7, O8, O9, and O9.5 for clarity.