2020ApJ...890...69L

ZTF J153932.16+502738.8 (ZTFJ1539) is an eclipsing double-white-dwarf system with an orbital period of 6.91 minutes, and a significant source for LISA detection of gravitational waves. However, the massive white dwarf (WD), with a mass of about 0.61 M_☉, has a high effective temperature (48,900 K), and the lower-mass WD, with a mass of about 0.21 M_☉, has a low effective temperature (<10,000 K). This discrepancy challenges the popular theory of binary evolution. We investigate the formation of ZTFJ1539 via nova and Algol scenarios. Assuming that the massive WD in ZTFJ1539 just experiences a thermonuclear runaway, the nova scenario can explain the effective temperatures of the two WDs in ZTFJ1539. However, in order to enlarge a semi-detached orbit of about 4-5 minutes to a detached orbit of about 7 minutes, the nova scenario needs a much higher kick velocity of about 200 km s^–1 during nova eruption. The high kick velocity can result in a high eccentricity of about 0.2-0.6. The Algol scenario can also produce ZTFJ1539 if we take a high efficient parameter for ejecting the common envelope and enhance the mass-loss rate via stellar wind trigger by tidal effect.