Observed and predicted I(4686)/I(Hβ) relative nebular line intensities as a function of metallicity. Observations of low-metallicity star-forming galaxies are shown as red plusses, and different measurements of the NW region of I Zw 18 with blue diamonds. The yellow line shows the linear regression to the data points. Assuming a default value of q = 2 × 10¹⁰ photon erg⁻¹, the empirical L_X/SFR–O/H relations of Douna et al. (2015) and Brorby et al. (2016) translate into He II intensities shown by the blue and magenta solid lines, respectively. The black lines show the predicted He II intensity adopting L_X/SFR predicted from the XRB synthesis models described in Sect. 3 for a constant SFR over 10 Myr (dotted) and 0.1 Gyr (solid), and the same value of q. The blue dashed line differs from the solid line by assuming a value of q that is a factor of two higher.