Comparison of EWs measured in individual galaxies (black stars) and stacks (coloured symbols) with predictions from BPASS models. Left panel: EWs of He II and C III], and right panel: EWs of He II and O III]. Also shown are EW measurements from Nanayakkara et al. (2019). The measurements from individual sources in our sample are in close agreement with those of Nanayakkara et al. (2019). The BPASS models underpredict the He II EW, but subsolar metallicity models can reproduce the C III] EW. The O III] EWs are also underpredicted at all metallicities for the majority of sources. Solar metallicity models underpredict the EWs by several orders of magnitude, and therefore we can conclude that based on the observed UV line EWs, He II-emitting galaxies tend to favour subsolar metallicities. To properly account for the missing He II ionising photons, additional mechanisms such as stripped stars (Götberg et al. 2018, 2019) or X-ray binaries (Schaerer et al. 2019) may be needed.