Distribution of orbital period and luminosity of the OB star companion from BH+OB star binaries that can produce X-ray luminosity 10³¹–10³⁵ erg/s through wind accretion, for two values of the viscous heating parameter δ = 0.001 (left panel) and 0.5 (right panel). The colour bar gives the predicted fraction of X-ray-faint BH+OB star binaries in each pixel. The sum of the pixels in each panel is equal to unity. VFTS 243 (Shenar et al. 2022b) is shown by a black star. Two more potential BH+OB star binaries are marked with thin and thick diamonds (Shenar et al. 2022b). A further six SB1 systems with unseen companions (having mass above 2.35 M_⊙) are marked by white circles (Table 1; data from Shenar et al. 2022b).