Basic SEDs used for the photoionisation models: The three ULX spectra (DIS, BMC, and KAA), normalised to the same X-ray luminosity L_X, are shown by the red, blue, and green lines, respectively. For comparison the pure stellar SED, labelled STE and shown by the grey line, is scaled to our standard value of L_X/SFR = 7.9 × 10³⁹ erg s⁻¹/(M_⊙ yr⁻¹); see text. The SED of I Zw 18 (LEB model) from Lebouteiller et al. (2017) is shown in yellow and scaled to the same stellar UV flux at ∼10 eV as STE, resulting in an L_X higher by a factor ∼2.8 compared to the ULX spectra. The vertical dashed lines mark the ionisation potentials of hydrogen and helium (13.6 eV and 54 eV, respectively). The grey shaded area highlights the 0.5−8 keV range of the spectra used for L_X.