2022ApJ...925...89C

Double white dwarf (DWD) binaries are important for studies of common-envelope (CE) evolution, Type Ia supernova progenitors and Galactic sources of low-frequency gravitational waves. PTF J0533+0209 is a DWD system with a short orbital period of P _orb ∼ 20 minutes and thus a so-called LISA verification source. The formation of this system and other DWDs is still under debate. In this paper, we discuss the possible formation scenarios of this binary and argue that it is not likely to have formed through CE evolution. Applying a new magnetic-braking prescription, we use the MESA code to model the formation of this system through stable mass transfer. We find a model that can well reproduce the observed WD masses and orbital period but not the effective temperature and hydrogen abundance of the low-mass He WD component. We discuss the possibility of using H flashes to mitigate this discrepancy. Finally, we discuss the future evolution of this system into an AM CVn binary such as those that will be detected by spaceborne GW observatories like LISA, TianQin, and Taiji.