Distribution of CO WD and He WD masses for different BPS sets. The mass transfer between the CO WD and the He WD is always unstable for the systems above the dashed line (q = M_He/M_CO> 2/3), while it is always stable below the dot-dashed line (Marsh et al. 2004). The systems located at the right side of the triple-dot-dashed line have a mass of ≥1.378 M_⊙. Three extremely low-mass (ELM) WD systems (J0751, J1741 and J1257+5428), whose component masses were accurately measured, are clearly located in the region allowing stable mass transfer (Kulkarni & van Kerkwijk 2010; Marsh et al. 2011; Kilic et al. 2014). The filled triangles represent the systems from the ELM survey that may merge within 10 Gyr, where i = 60° is assumed for systems with unknown inclinations (Kilic et al. 2012).