Evolutionary tracks starting from the ZAMS at metallicity Z = 0.001 of single stars (left) and some secondary stars in binary systems (right) with ZAMS masses of 10, 15, 20, 25, 35, 50, 60, 80, and 100 M_⊙ and metallicity Z = 0.001. The central and right-hand plots show the secondary stars in the CHE10preMS and NoCHEpreMS models. The colour-map shows the wind optical depth during the stars’ evolution. The blue and red-filled regions indicate the threshold within which the luminosity and temperature are necessary to classify a star as a WR or a RSG.