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Hot subdwarf B stars (sdBs) are evolved core helium-burning stars with verythin hydrogen envelopes. In order to form an sdB, the progenitor has to losealmost all of its hydrogen envelope right at the tip of the red giant branch.In binary systems, mass transfer to the companion provides the extraordinarymass loss required for their formation. However, apparently single sdBs existas well and their formation is unclear since decades. The merger of heliumwhite dwarfs leading to an ignition of core helium-burning or the merger of ahelium core and a low mass star during the common envelope phase have beenproposed. Here we report the discovery of EC 22081-1916 as a fast rotating,single sdB star of low gravity. Its atmospheric parameters indicate, that thehydrogen envelope must be unusually thick, which is at variance with the He-WDmerger scenario, but consistent with a common envelope merger of a low-mass,possibly substellar object with a red-giant core.

The fast rotating, low gravity subdwarf B star EC 22081-1916 - Remnant of a common envelope merger event