Q_HeII vs. M⋆ for our sample. The colours correspond to the gas-phase metallicity. The oblique lines represent the Q_HeII predictions from the BPASS models, scaled linearly with stellar mass. Models with interacting binaries are shown as dashed lines. Those without them are shown as solid lines. In all cases we show the metallicities of Z = 10⁻³, 10⁻⁴, and 10⁻⁵. Additionally, we present models adopting a standard Salpeter IMF (Salpeter 1955) with an upper mass limit of $M_{\mathrm{up}}=100$ M_⊙ (red lines) and models assuming a top-heavy IMF with M_up = 300 M_⊙ (blue lines). SBS 0335–052E (Kehrig et al. 2018) is included as a black square, which presents 12 + log(O/H) = 7.3.