Positions of HeS stars (at the minimum luminosity along their evolutionary tracks, see the text) and their progenitors in the HRD. The ZAMS masses are color-coded. The crosses mark the progenitors of HeS stars at the beginning of the RLOF, and the filled circles show HeS stars descending from them. The progenitor’s and descendant’s symbols of the same color belong to the same evolutionary tracks, i.e., red circles denote the descendants of red crosses, yellow circles the descendants of yellow crosses, and so on. Dashed magenta lines bound the regions occupied by the Galactic WN stars, HeS stars, and precursors of the latter. In the upper right corner, dashed blue lines indicate the limits of case B mass exchange and stable quasi-conservative mass exchange. The black symbols in the diagram indicate stars with ZAMS mass 28 M_⊙, which produce WR stars. Thick and thin solid and dashed lines show the evolutionary tracks of the primaries of binaries with ZAMS masses of 22, 15, and 7, and 5 M_⊙, and initial periods of 5 (case A mass exchange), 550 (case B), 100 (case B), and 50 days (case B), respectively, illustrating the formation paths of HeS stars. The thin dotted lines are the lines of equal stellar radii. The gray ellipses depict the locations of sdB and sdO-stars (from Heber 2016, Fig. 1).