A seven-Earth-radius helium-burning star inside a 20.5-min verification binary of GWs

Abstract Binary evolution theory predicts that the second common envelope (CE) ejection can produce low-mass (0.32-0.36 M⊙) subdwarf~B (sdB) stars inside ultrashort-orbital-period binary systems as their helium cores are ignited under nondegenerate conditions. With the orbital decay driven by radiation of gravitational waves, the minimum orbital period that an sdB binary formed in this channel can reach before the Roche-lobe overflow is ~20 minutes. However, only four sdB binaries with orbital periods below an hour have been reported so far, while none of them has an orbital period approaching the above theoretical limit. This leads to an unclear role of the second CE ejection in forming ultracompact sdB binaries and a blocked link between detached helium-burning-star binaries and gravitational-wave sources, AM CVn stars. Here we report the discovery of a 20.5-minute-orbital-period ellipsoidal binary, TMTS J052610.43+593445.1, in which the visible helium-burning sdB star is being tidally deformed by an invisible carbon-oxygen white dwarf (WD) companion. This sdB star is inferred to have a mass of ~0.33 M⊙, approaching that of the helium-ignition limit. Owing to the low-mass sdB star, WD companion, and extremely short orbital period, this system provides the first clear case to justify the formation of sdB binaries through the nondegenerate channel of second CE ejection. In particular, the radius of this low-mass sdB star is only 0.066 R⊙, about seven Earth radii, representing the most compact nondegenerate star ever known. As the shortest-orbital-period single-degenerate detached binary discovered up to date, TMTS J052610.43+593445.1 provides a crucial bridge to map the binary evolution scheme from the second CE ejection to the formation of AM CVn stars having a helium-star donor, and will serve as a crucial verification binary of space-borne gravitational-wave detectors in the future.