Observed orbital periods, log P_orb, and mass ratios, m_CO/m_star, for low (and intermediate) mass single-degenerate binaries. More specifically: WD + MS stars from the ZTF catalogue of Brown et al. (2023; ZTF) and Holberg et al. (2013; Sirius-like); sdO/B + MS stars from van Roestel et al. (2018), Maxted et al. (2014) and ten Kepler systems (see main text); sdO/B + Be stars, which are separated into candidate systems (candidate sdO/B + Be), confirmed sdO + Be, and γCas-like systems, as in Wang et al. (2023). Markers report the reference data points without uncertainties. Solid, colored contours are the correspondent loci predicted by our calculations to be promising for GW progenitor systems via stable MT and follow the same notation as in Fig. 14.