H_α line profiles at different phases φ (top row) with corresponding mean-subtracted dynamical spectra (middle row) and radial velocity curves (bottom row) of the BH or strB companion. The x_obs-axis describes the frequency shift from line centre expressed in velocity space. The first three columns to the left display the solution for the three different models, PB_SBH (primary: B star, secondary: BH), PST_SBE01 (primary: stripped star, secondary: Be-star, model 1), PST_SBE02 (primary: stripped star, secondary: Be-star, model 2), with H_α line profiles at two distinct phases from the actual observations indicated by the black dashed and dotted lines. The right column additionally displays the corresponding observed (see Shenar et al. 2020) H_α line profiles, mean-subtracted dynamical spectrum and radial velocity curves of LB-1, where we distinguish between the true ones (e.g., from using disentangling methods) indicated in blue and those obtained from the barycentric method applied to the H_α line wings (red). To reduce the noise, all observed line profiles have been convolved with a Gaussian filter of width 10 km s⁻¹. The bottom panels display the radial velocity curves of the secondary when calculated from the barycentric method applied to the H_α line profile wings (red crosses) and when using the true orbital parameters (blue crosses). Additionally, we display the corresponding radial velocity curves as found by Shenar et al. (2020) and Liu et al. (2019) using disentangling techniques (blue solid line) and the barycentric method (red solid lines), respectively. At the top, the (phase-averaged) equivalent width evaluated in velocity space in units of 100 km s⁻¹ and the inclination used within the synthetic-spectra calculations are indicated.