Optical depth at the base of the wind of a stripped-envelope star as a function of its metallicity and luminosity, obtained from Eq. (6) (based on the optical depth derivation, mass-luminosity, and radius-luminosity relations of Langer 1989). The mass loss rate of WN stars is used, which depends on metallicity and luminosity, as in Eq. (1). The star symbols represent the lowest luminosity observed for WN stars in the SMC, LMC, and Milky Way, according to Shenar et al. (2020). Error bars with a width of 0.05 dex are included as these values are rounded. The lines represent the solutions of Eq. (6) for constant optical depth, including those that best fit the observed values of the observed minimum luminosities of WN stars.