An extreme helium star (abbreviated EHe) is a low-mass supergiant that is almost devoid of hydrogen, the most common chemical element of the Universe.
Since there are no known conditions where stars devoid of hydrogen can be formed from molecular clouds, it is theorized that they are the product of the mergers of helium-core and carbon-oxygen core white dwarfs.
[1] The first known extreme helium star, HD 124448, was discovered in 1942 by Daniel M. Popper at the McDonald Observatory near Fort Davis, Texas, United States.
The chemical composition of these stars implies that they have undergone both hydrogen and helium burning at some stage of their evolution.
[3] Examination of element abundances from seven EHe stars agreed with those predicted by the DD model.