Yields of H I-, He I-, and He II-ionizing photons (I_ion,H I, I_ion,He I, and I_ion,He II respectively). We show these yields as functions of metallicity. The photon yields from stripped stars are shown in green and those from massive stars in pink (using models from Starburst99). Panels a, b and c: yield of H I-, He I-, and He II-ionizing photons, respectively. The values are presented in Table A.1. (Z_⊙ ≡ 0.014, Asplund et al. 2009.)