Herzsprung-Russel diagram of the Galactic WN stars, and the LBV AG Car. Red/blue symbols indicate observed HR diagram positions of WN stars from Hamann et al. (2006) (blue: with hydrogen (X > 0.05); red: hydrogen-free (X < 0.05)). Black symbols indicate the HR diagram positions of AG Car throughout its S Dor Cycle from 1985–2003, according to Groh et al. (2009b). Large symbols refer to stars with known distances from cluster/association membership. The symbol shapes indicate the spectral subtype (see inlet). Arrows indicate lower limits of T_⋆ for stars with strong mass loss. The observations are compared to stellar structure models with/without hydrogen from this work (blue/red lines, cf. Fig. 6), and for AG Car (X = 0.36, black line).