Nature of the hidden companions in the 51 SB1 binaries analysed here. Upper panel: masses of the previously “hidden” companions in the newly uncovered SB2 systems as a function of the orbital period. In total, 28 out of the 51 SB1 systems in our sample are unambiguously found to host non-degenerate companions, though 31 symbols are plotted due to the presence of higher-order multiples. For the tertiary of VFTS 64, the outer period and inner binary mass are denoted. For the quadruples (VFTS 120 and 702), only the binary pairs are considered, since the outer period is not known, and a primary mass of M₁ = 20 ± 10 M_⊙ is assumed for VFTS 702B for plotting purposes. The symbols mark the medians of the distributions, and the error bars denote the 68% confidence intervals (see Sect. 4.1). Lower panel: as upper panel, but for the SB 1 and uncertain SB 1 and SB2 systems (SB 1:, SB2:), which could in principle host a BH. Shaded region depicts the mass regime M₂ > 3 M_⊙ in which BHs are expected to reside. Along with the confirmed O+BH binary VFTS 243, two other OB+BH candidates are highlighted (VFTS 514 and 779).