tokens: component of this line and the corresponding tomogram are presented in Fig. 8 . Meanwhile, the same line is treated as an absorption feature on the right-hand side. The emission component of the line basically repeats the pattern observed in H α. The observed trailed spectrum is faint, but the program picks up the signal correctly, reflecting it in the reconstructed spectrum. The low-velocity component definitely dominates, and hence a compact spot materializes close to the L1 point, in contrast with the diffuse appearance in the H α case. The HVC is hardly traceable in the observed trailed spectra, but apparently some emission comes from here too. Hence, a coma-like extension in the velocity map is visible 5 SYSTEM PARAMETERS The average mass of WDs in CVs above the period gap is M WD ( P orb > 3 h) = 0.86(20) M ⊙ (Zorotovic et al. 2011 ). For simplicity, we adopted M WD = 0.8 M ⊙ . Taking into account the semi-empirical relation of secondary type versus orbital period, we expect a secondary with a mass of M 2 ≈ 0.3 M ⊙ and spectral type of M3V–M4V (Knigge 2006 ). The expected mass ratio and separations are q ≡ M 2 / M WD ≈ 0.4 and a = 1.23 R ⊙ , respectively. The accretion-disc truncation radius 4 is R ⊙ . The corresponding primary, secondary, and L1 orbital velocities are 117, 292, and 129 km s −1 , respectively (see Fig. 9 ). Figure 9. A geometric model of BG Tri. The centre of mass (x) and corresponding Lagrangian points are marked. Velocities of the L1 point, the disc outflow area (green), and the WD are denoted. The LVC, which is emitted from the irradiated hemisphere of the secondary star, has a velocity exceeding that of the L1 point and less than the orbital velocity of the star. There are numerous flux measurements of BG Tri available in the public domain. We fetched all available data from VizieR 5 (Ochsenbein, Bauer Marcout 2000 ). We revised the available data, since some measurements are erroneous (e.g. the SDSS data are not correct because the object is too bright), and compiled those that passed scrutiny in Table 2 . Selected data are plotted using circles in the bottom panel of Fig. 10 , after the interstellar reddening correction of E ( B − V ) = 0.03 (Green et al. 2015 ). Using the Gaia distance of d = 334(8) pc, we can calculate the luminosity of different components. In particular, even a hot 50 kK white dwarf will have a negligible contribution to the optical flux. A secondary M3–4 V Roche lobe filling star has some insignificant influence in the IR. A comparison of the observed fluxes with those expected from different components confirms that the flux in the entire wavelength range is formed mostly by the accretion disc. The dashed lines in the bottom panel of Fig. 10 indicate a simple accretion-disc spectrum as a composition of multiple blackbodies, from concentric rings with a corresponding distribution of temperatures throughout the stationary optically thick disc (La Dous 1989 ). Since the observed flux would
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