Optical spectrum of THA 15−31 as recorded with X-shooter (black) is fitted (red) by combining the spectrum of the cool giant (M6III, green spectrum) and that of the hot companion (blue line). The latter is a bremsstrahlung spectrum taken to approximate the mix of the accretion disk, the central star, and possibly some RG wind ionized by them. The spectra of the M6III cool giant and of the hot companion are reddened by E_B − V = 0.38 following the Fitzpatrick (1999) reddening law for the standard R_V = 3.1 case. The inset is a zoomed-in view of the head of the Balmer series and the Balmer jump.