Optical depth (left, color-coded) and surface hydrogen abundance (right) of our single star models as function of initial mass and normalized time, during their main-sequence (lower) and core helium burning (upper) evolution. The transition line from an optically thin to an optically thick wind is marked by a solid black line labeled with “1.500”. The unlabeled solid black lines divide the different considered WR phases as labeled in each region. In the right plot, threshold values of the surface hydrogen abundances in steps of ΔX_H = 0.1 are indicated by thin solid lines, which have dark red color for the range X_H = 0.8 − 0.5 and brown color for X_H = 0.4 − 0.1. The borderlines of X_H = 0.4 and X_H = 0.0 are marked by solid black lines, as well as the borderline beyond which stellar models have a surface carbon mass fraction in excess of X_C = 0.01, which is our threshold for defining WC stars.