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Binary evolution theory predicts that the second common envelope (CE)ejection can produce low-mass (0.32-0.36 Msun) subdwarf B (sdB) stars insideultrashort-orbital-period binary systems, as their helium cores are ignitedunder nondegenerate conditions. With the orbital decay driven bygravitational-wave (GW) radiation, the minimum orbital periods of detached sdBbinaries could be as short as ~20 minutes. However, only four sdB binaries withorbital periods below an hour have been reported so far, while none of them hasan orbital period approaching the above theoretical limit. Here we report thediscovery of a 20.5-minute-orbital-period ellipsoidal binary, TMTSJ052610.43+593445.1, in which the visible star is being tidally deformed by aninvisible carbon-oxygen white dwarf (WD) companion. The visible component isinferred to be an sdB star with a mass of ~0.33 Msun, approaching that ofhelium-ignition limit, although a He-core WD cannot be completely ruled out. Inparticular, the radius of this low-mass sdB star is only 0.066 Rsun, aboutseven Earth radii, possibly representing the most compact nondegenerate starever known. Such a system provides a key clue to map the binary evolutionscheme from the second CE ejection to the formation of AM CVn stars having ahelium-star donor, and it will also serve as a crucial verification binary ofspace-borne GW detectors in the future.

A seven-Earth-radius helium-burning star inside a 20.5-min detached binary