Evolution of surface luminosity as a function of age, between the He-ZAMS and core carbon depletion, for helium star models with different initial masses, coloured by surface helium abundance. Each panel represents a set of models with different metallicity, from 0.01 (top left) to 0.04 (bottom). Arrows on the right side of the panel with Z = 0.01 represent the luminosities of observed WC stars in the LMC, inferred from the sample of Bartzakos et al. (2001), assuming a bolometric correction of M_bol = 4.5 (e.g. Moffat 1989; Smith et al. 1994), and have typical uncertainties of 0.3 dex. Arrows on the right side of the panel with Z = 0.02 represent the luminosities of observed WC stars in the Galaxy, taken from Sander et al. (2019). The thin horizontal dashed lines represent the minimum luminosity at which carbon-rich models are formed at each metallicity, . The solid black lines indicate the moment at which stellar models end core helium burning, and have surface oxygen abundance higher than 0.05. The horizontal thick dotted lines indicate the minimum luminosity at which the optical depth of nitrogen-rich stripped-envelope stars is expected to be higher than 1.45, according to Eq. (7). The horizontal thick dash-dotted lines indicate the minimum luminosity at which the optical depth of carbon-rich stripped-envelope stars is expected to be higher than 1.45, according to Eq. (8).