Observed and predicted I(4686)/I(Hβ) relative nebular line intensities as a function of the Hβ equivalent width. Observations are the same as in Fig. 1. Blue crosses show the predictions from the BPASS models of Xiao et al. (2018), which fail to reproduce the observed He II λ4686 intensities. Predictions for simple stellar populations of different metallicities (shown by the solid lines), derived from the combination of BPASS + XRB models, show a fair agreement with the observations.