Optical spectrum of Ho II X-1 taken with the Subaru telescope. The spectrum is normalised to the stellar model continuum and corrected for extinction. In blue, the observations, in dotted green the synthetic stellar spectrum from the PoWR model for a B0.5I supergiant (T_* =  26 k K, log g =  3.1 [cm s⁻²] and log L/L_⊙ =  5.3), in dotted purple the CLOUDY nebular model for an X-ray ionising source modelled with diskir, and in dotted red the combined PoWR and CLOUDY spectra. The most relevant features are identified in black, while those identified in green correspond to high ionisation stages of iron and calcium overpredicted by the CLOUDY model (Sect. 3.3).