  Type Ia supernovae are cosmic distance indicators, and the main source of
iron in the Universe, but their formation paths are still debated. Several
dozen supersoft X-ray sources, in which a white dwarf accretes hydrogen-rich
matter from a non-degenerate donor star, have been observed and suggested as
Type Ia supernovae progenitors. However, observational evidence for hydrogen,
which is expected to be stripped off the donor star during the supernova
explosion, is lacking. Helium-accreting white dwarfs, which would circumvent
this problem, have been predicted for more than 30 years, also including their
appearance as supersoft X-ray sources, but have so far escaped detection. Here
we report a supersoft X-ray source with an accretion disk whose optical
spectrum is completely dominated by helium, suggesting that the donor star is
hydrogen-free. We interpret the luminous and supersoft X-rays as due to helium
burning near the surface of the accreting white dwarf. The properties of our
system provides evidence for extended pathways towards Chandrasekhar mass
explosions based on helium accretion, in particular for stable burning in white
dwarfs at lower accretion rates than expected so far. This may allow to recover
the population of the sub-energetic so-called Type Iax supernovae, up to 30% of
all Type Ia supernovae, within this scenario.
