Orbital period, P, of the binary system as a function of mass ratio q, assuming a present-day period of P_r ∼ 40 days and the two estimates of the current mass ratio from spectroscopy alone (pink; Bodensteiner et al. 2020, 15 ± 3) and combined spectroscopy and optical interferometry (purple; Klement et al. 2025, 15.7 ± 1.1), with their respective uncertainties (lighter colored bands around the respective line). HR6819 properties from these two works are also reported as scatter points, with their error bars on the mass ratio (the error bars on the period are too small to be visible). MT is assumed to be either fully conservative (solid) with ϵ = 1 or fully non conservative (dashed) with β = 1. Circles represent a selection of detached and semidetached systems from the S_B⁹ catalogue, as in Figure 15 from Bodensteiner et al. (2020); see also their Appendix D.