  Type Ia supernovae (SN Ia) are the most important standard candles for
measuring the expansion history of the universe. The thermonuclear explosion of
a white dwarf can explain their observed properties, but neither the progenitor
systems nor any stellar remnants have been conclusively identified.
Underluminous SN Ia have been proposed to originate from a so-called
double-detonation of a white dwarf. After a critical amount of helium is
deposited on the surface through accretion from a close companion, the helium
is ignited causing a detonation wave that triggers the explosion of the white
dwarf itself. We have discovered both shallow transits and eclipses in the
tight binary system CD-30 11223 composed of a carbon/oxygen white dwarf and a
hot helium star, allowing us to determine its component masses and fundamental
parameters. In the future the system will transfer mass from the helium star to
the white dwarf. Modelling this process we find that the detonation in the
accreted helium layer is sufficiently strong to trigger the explosion of the
core. The helium star will then be ejected at so large a velocity that it will
escape the Galaxy. The predicted properties of this remnant are an excellent
match to the so-called hypervelocity star US 708, a hot, helium-rich star
moving at more than 750 km/s, sufficient to leave the Galaxy. The
identification of both progenitor and remnant provides a consistent picture of
the formation and evolution of underluminous type Ia supernovae.
