A hot subdwarf-white dwarf super-Chandrasekhar candidate supernova Ia
  progenitor
  Supernova Ia are bright explosive events that can be used to estimate
cosmological distances, allowing us to study the expansion of the Universe.
They are understood to result from a thermonuclear detonation in a white dwarf
that formed from the exhausted core of a star more massive than the Sun.
However, the possible progenitor channels leading to an explosion are a
long-standing debate, limiting the precision and accuracy of supernova Ia as
distance indicators. Here we present HD265435, a binary system with an orbital
period of less than a hundred minutes, consisting of a white dwarf and a hot
subdwarf -- a stripped core-helium burning star. The total mass of the system
is 1.65+/-0.25 solar-masses, exceeding the Chandrasekhar limit (the maximum
mass of a stable white dwarf). The system will merge due to gravitational wave
emission in 70 million years, likely triggering a supernova Ia event. We use
this detection to place constraints on the contribution of hot subdwarf-white
dwarf binaries to supernova Ia progenitors.
