Observed orbital periods, log P_orb, and mass ratios, i.e. m_CO/m_star, for high (and intermediate) mass X-ray binaries and quiescent OB + BH systems. The markers report data points (uncertainties are neglected) listed in a) Table 1 for HM(XR)Bs with BHs (black stars), with the corresponding labeled names, and b) Table 1 from Tomsick & Muterspaugh (2010) for HM(XR)Bs with NS (red stars). We also report the Casares et al. (2014) system (blue star). The triangles show the sample of confirmed sdO + Be stars in Table 9 of Wang et al. (2023). The solid, colored contours are the correspondent loci predicted by our calculations to be promising for GW progenitor systems via stable MT, within the assumption of purely isotropic re-emission, β = 1. Every contour is labeled with the associated fixed donor’s initial mass, m_d, i. The dotted, colored contours show the same loci (m_d, i/m_a, i, log P_i) within the assumption of an efficiency for mass outflow via L₂ such that υ = 0.5.