  Binary evolution theory predicts that the second common envelope (CE)
ejection can produce low-mass (0.32-0.36 Msun) 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
gravitational-wave (GW) radiation, the minimum orbital periods of detached sdB
binaries could be as short as ~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. Here we report the
discovery of a 20.5-minute-orbital-period ellipsoidal binary, TMTS
J052610.43+593445.1, in which the visible star is being tidally deformed by an
invisible carbon-oxygen white dwarf (WD) companion. The visible component is
inferred to be an sdB star with a mass of ~0.33 Msun, approaching that of
helium-ignition limit, although a He-core WD cannot be completely ruled out. In
particular, the radius of this low-mass sdB star is only 0.066 Rsun, about
seven Earth radii, possibly representing the most compact nondegenerate star
ever known. Such a system provides a key clue to map the binary evolution
scheme from the second CE ejection to the formation of AM CVn stars having a
helium-star donor, and it will also serve as a crucial verification binary of
space-borne GW detectors in the future.
