PoWR model spectra of individual stars during the post-main-sequence (core-helium burning) phase of the same evolutionary model (M_ini = 131 M_⊙, actual mass between 93−106 M_⊙). The model’s approximate position is shown on the HR diagram in Fig. 1 by the last point (pMS phase) at around log(T_eff/K) ∼ 5.14 and log(L/L_⊙) ∼ 6.7. Top: Optical range. Bottom: UV range. Nominal mass loss rate and no clumping was assumed, to be consistent with previous work (see Paper II, and the discussion of the caveats in Sect. 5.2). Y_C indicates core-helium abundance and thus the evolutionary progress during the post-MS. The earliest model (red straight line, Y_C = 0.8, i.e. just burned about 20% of its helium in the core) shows prominent emission only in helium, while the mid (blue straight line, Y_C = 0.5) and late (black straight line, Y_C = 0.1, corresponding to the bottom right panel of Fig. 2) ones develop carbon and oxygen lines too. (The Cl and S lines are modelling side-effects, we do not expect them to show up in observations.) However, the distinguishing emission line C IIIλ5696 Å which serves as the basis of classifying a stars as WC (Crowther et al. 1998) is missing in all three models. So are the nitrogen emission lines that would categorize a star as late-type WN (Crowther et al. 1995; Smith et al. 1996; Crowther & Walborn 2011). If observed, therefore, these stars would be identified as early-type WN (i.e. WN2) and then during most of the core-helium burning, as WO – more precisely, WO 2 evolving to WO 1.