Hertzsprung-Russell diagram for the evolution of massive stars. Theoretical tracks at solar metallicity are taken from Ekström et al. (2012) for stars rotating initially at 40% of their critical velocity. The size of the markers is proportional to the amplitude of the SLF variability (Sect. 5.3). The variables of the α Cygni class are indicated by outer circles, and the thick line represents the Humphreys-Davidson limit. The uncertainty in the spectroscopic temperature is illustrated by the error bar on the upper right.