Hertzsprung–Russell diagram of Galactic WN stars with Gaia parallax. The filling color reflects the surface composition (red: with hydrogen; green: hydrogen-free), while the symbol shape refers to the spectral subtype (see caption of Fig. 2). The binaries and runaways are encircled. The labels refer to the WR catalog numbers; if printed with slantedfont, their Gaia measurement is flagged with a significant astrometric excess noise (see Sect. 3). A little arrow indicates that for this particular star the indicated temperature is only a lower limit, because of the parameter degeneracy discussed in the text (Sect. 5.2). The vertical error bars reflect only the 1σ margin of the Gaia distances. Just for orientation, the ZAMS are indicated for solar composition and for pure helium stars, respectively.