Periapsis R_p in units of the gravitational radius from the mass–radius relation of WDs (shown in panel a) and orbital eccentricity (shown in panel b). The results of King (2020) are shown in black circles for GSN 069. For eRO-QPE1 and eRO-QPE2, M₁ is estimated by 0.025%M_stellar (Reines & Volonteri 2015). The radii of ISCO for Schwarzschild BHs (a = 0) and extreme-rotating BHs (a = 0.998) are also illustrated in black lines. Panel a: Mass–radius relation of WDs is taken from Zalamea et al. (2010). The case of WDs companion for RXJ1301.9+2747 and eRO-QPE2 is only possible for extreme low-mass WDs or Kerr BHs. Panel b: unless the eccentricity is very high, the mass transfer can be carried out stably.