Top panel: DSS2 coloured image of 2° × 1.8° including 4U 1812−12 (white rectangle), which is located in the outskirts of the H II nebula Sharpless 54. A zoom-in on the target region using our Hα narrow-band imaging is shown together with the position of the slit in the spectroscopic data. A 120 arcsec region covered by the slit along the spatial axis and corresponding to the 2D spectrum (middle panel) is represented by a yellow rectangle, while the position of the system is indicated by a green circle. For better visualization, the closest region to the target is magnified further. Middle panel: zoom-in on 2D spectrum in the Hα region, covering 120 arcsec in the spatial direction. The position of 4U 1812−12 is indicated by a dashed red line. The vertical white stripes denote the different background regions considered for the overall (A) and nebula-free (B) spectra (see text). Intensity is indicated by a colour scale from 0 (deep blue) to 160 (bright yellow) counts. Bottom panel: average spectra obtained by considering the A (red) and B (black) background regions. The grey-shaded bands indicate spectral regions affected by interstellar or telluric features.