  We investigate the contribution of different formation scenarios for type Ia
supernovae in elliptical galaxies. The single degenerate scenario (a white
dwarf accreting from a late main sequence or red giant companion) is tested
against the double degenerate scenario (the spiral-in and merging of two white
dwarfs through the emission of gravitational wave radiation). We use a
population number synthesis code incorporating the latest physical results in
binary evolution, and allowing to differentiate between certain physical
scenarios (e.g. description of common envelope evolution) and evolutionary
parameters (e.g. mass transfer efficiency during Roche lobe overflow). The
obtained theoretical distributions of the delay times of type Ia supernovae are
compared to those which are observed, both in morphological shape and absolute
number of events. The critical dependency of these distributions on certain
parameters is used to constrain the values of the latter. We find that the
single degenerate scenario alone cannot explain the morphological shape of the
observational delay time distribution, while the double degenerate scenario (or
a combination of both) can. Most of these double degenerate type Ia supernovae
are created through a normal quasi-conservative Roche lobe overflow followed by
a common envelope phase, not through two successive common envelope phases.
This may cast doubt on the use in other studies of analytical formalisms to
determine delay times. In terms of absolute number, theoretical supernova Ia
rates in old elliptical galaxies lie a factor of at least three below the
observed ones. We propose a solution involving the effect of rotation on the
evolution of intermediate mass binaries.
