Spectral energy distribution (SED) of the ULX Ho II X-1 from UV to optical. Upper panel A: The blue line shows the data extracted from the observations with HST COS (UV), GTC OSIRIS and Subaru FOCAS (both in the optical). The blue boxes display photometric measurements from the HST ACS SBC F165LP in the UV and from HST WFPC2 in the optical (Sect. 4.1). The black lines at the bottom represent the filter wavelength ranges, each of them identified by the name of the filter. The red line indicates the synthetic PoWR SED for a B0.5I supergiant with T_* =  26 k K, log g =  3.1 cm s⁻² and log L/L_⊙ =  5.3. The red hatched boxes show the modelled photometry. Bottom panel B: The blue boxes are the photometric measurements of the Ho II X-1 counterpart from HST filters without any correction for the nebular contribution. The purple line shows the synthetic CLOUDY SED for a nebula photoionised by an X-ray source, as is described by a diskir model (Sect. 3.2, Table 2), and with a luminosity of L_bol =  1.4 × 10⁴⁰ erg s⁻¹. The green line represents the synthetic PoWR SED for a B0.5I supergiant with T_* =  26 k K, log g =  3.1 [cm s⁻²] and log L/L_⊙ =  5.3. The red line is the combination of both PoWR and CLOUDY models, and the red-hatched boxes represent the modelled photometry for the combined model.