Time evolution of surface properties of the evolutionary model with M_ini = 131 M_⊙ after the terminal-age main sequence (TAMS). Reproduced from Szécsi (2016, their Fig. 4.13), adding information about our three post-main-sequence PoWR models scrutinized in Sect. 2.2 and Fig. 4. Black star-symbols mark their positions on the central helium mass fraction: Y_C = 0.8 (early), Y_C = 0.5 (mid), Y_C = 0.1 (late). The rudimentary classification of Szécsi (2016, see their Sect. 4.5.1, based on Georgy et al. 2012) is surpassed by the spectral-line-based classification scheme (see also Fig. 4 showing all the optical emission lines), meaning that chemically homogeneously evolving stars at I Zw 18 metallicity do not spend significant time in the WC phase, proceeding from their main-sequence O-star phase to WN and then directly to WO.