Production rates of He II- and He I-ionizing photons (normalized to that of H-ionizing photons in the upper two panels and relative to each other in the bottom panel) as a function of the stellar-population age for different GALSEVN models. Solid orange and red lines denote GALSEVN Pop III (Z = 10⁻¹¹) and extremely metal-poor Pop II (Z = 10⁻⁶), pure-binary models, while dotted lines denote the corresponding single-star models for comparison. Pop II pure-binary models for Z = 0.0001 and 0.001 are shown as dashed purple and blue lines for reference. All these models have the same zero-age Chabrier (2003) IMF. The green- and red-shaded regions indicate the areas covered by models with the three top-heavy IMFs with characteristic masses of 50 M_⊙, 100 M_⊙ and 200 M_⊙, as described in Sect. 2.3, for Z = 10⁻¹¹ and 10⁻⁶, respectively. These models are considered only up to the point where their absolute UV magnitude reaches M_UV = −14, beyond which the stars are too few and the curves too noisy (see Lecroq et al. 2024, for more details).