Two versions of the Hertzsprung-Russell diagram. In the top panel our sample of single O-type giants and supergiants is supplemented with the dwarf O star sample of Paper XIII and the sample of very massive Of and WNh stars by Paper XVII. We exclude the results of Paper XVII for the nine stars in common with this paper and adopted our results (see Sect. 3.4.3). Symbols and colors have the same meaning as in Fig. 6. The lower panel only contains the sample studied here. The symbol shapes in the lower panel show three categories of helium mass fraction, that is, not enriched (squares), moderately enriched (triangles), and enriched (stars). The symbol colors refer to their projected rotational velocity (see color bar on the right). Evolutionary tracks and isochrones are for models that have an initial rotational velocity of approximately 200 kms^-1 (Brott et al. 2011; Köhler et al. 2015). Iso-helium lines for different rotational velocities are from Köhler et al. (2015) and are color-coded using the color bar.