Emission rates of ionizing photons from stellar populations as a function of time. The contribution from stripped stars is shown in blue, while green represents the contribution from the massive main sequence and WR stars in the stellar population (models from STARBURST99, see Sect. 2.3). For reference, we show the predictions from BPASS models in which binary interactions are included using gray color. The solid lines correspond to the predictions from a population with solar metallicity, while the shaded regions of the same color represent the effect of lowering the metallicity, from solar down to Z = 0.0002 for the stripped stars (see Appendix A and Table A.1). Left column: emission rates from a co-eval stellar population with initially 10⁶ M_⊙ in stars, and right column: emission rates from a stellar population in which stars form at the constant rate of 1 M_⊙ yr⁻¹. Top, middle, and bottom rows: emission rates of H I-, He I-, and He II-ionizing photons, respectively (Q_0, pop, Q_1, pop, and Q_2, pop).