Optical emission lines predicted by our massive (M_ini = 59 M_⊙) and very massive (M_ini = 131 M_⊙), chemically homonegeously evolving, late-post-main-sequence models (classified as WO stars in this phase by Paper II; during the main-sequence evolutions, they were classified as early O type stars). In our fiducial population (Sect. 2.3), one or two such very massive stars are present with > 100 M_⊙; in our alternative population (Sect. 3.1), about six such stars with ∼40 M_⊙ and one with ∼80 M_⊙ are present, accounting for the measured He II ionizing flux. For the explanation of the legends, see Fig. 6. Note that we always apply the unclumped models’ predictions (i.e. black line).