Time evolution of optical depth parameter τ_wind (top panels) and electron scattering Eddington factor Γ_e (bottom panels, Eq. (2)) of the donor (black lines) for our two example models (Appendix A). The x-axis limits are the same as in Fig. A.1. The green vertical lines indicate the age at which the central hydrogen mass fraction of the donor is equal to 0.01, that is, the donor completes core hydrogen burning. The red and blue horizontal lines denote the calculated optical depth parameter of the observed LMC WN stars and Of or Of/WN stars, respectively, which have a surface hydrogen mass fraction of X_H ≥ 0.4 (Table 1).