UV line ratios determined for individual galaxies and stacks of He II emitters. The symbols are the same as in Fig. 6. The line ratios are compared with photo-ionisation models from star-forming galaxies including binary stars from Xiao et al. (2018) with three metallicities, Z = 0.0001, 0.002, and 0.02, and stellar ages 10⁶ (dot-dashed), 10⁷ (dashed), and 10⁸ (solid) years. Also shown are line ratios from Nanayakkara et al. (2019), Senchyna et al. (2017), Berg et al. (2016, 2018), and Patrício et al. (2016) for comparison, represented by the same coloured symbols as in Fig. 6. The majority of individual detections and stacks of Narrow and Faint He II emitters favour subsolar metallicities and log U_s >  −2, whereas the Broad emitters favour solar metallicities and lower log U_s values.