  Type Ia supernovae are key tools for measuring distances on a cosmic scale.
They are generally thought to be the thermonuclear explosion of an accreting
white dwarf in a close binary system. The nature of the mass donor is still
uncertain. In the single-degenerate model it is a main-sequence star or an
evolved star, whereas in the double-degenerate model it is another white dwarf.
We show that the velocity structure of absorbing material along the line of
sight to 35 type Ia supernovae tends to be blueshifted. These structures are
likely signatures of gas outflows from the supernova progenitor systems. Thus
many type Ia supernovae in nearby spiral galaxies may originate in
single-degenerate systems.
