  Close binaries consisting of a main sequence star and a white dwarf are
considered as candidates for Type~Ia supernova progenitors. We present
selfconsistent calculations of the time dependence of the structure of the main
sequence star, the mass transfer rate, and the orbit by means of a binary
stellar evolution program. In contrast to results based on simple estimates of
the mass transfer rate in systems of the considered type, our results allow for
the possibility that even systems with rather small initial white dwarf masses
(~ 0.7 M_sun) may produce Type Ia supernovae. We present results for two
different metallicities, Z=0.02 and Z=0.001. We find that for systems with the
lower metallicity, the mass transfer rates are on average five times larger
than in comparable system at solar metallicity. This leads to a systematic
shift of the supernova Ia progenitor population. Firstly, while for Z=0.02
donor star initial masses in supernova progenitor systems are restricted to the
range 1.6...2.3 M_sun, they are in the interval 1.4...1.8 M_sun at low Z.
Secondly, the initial white dwarf masses need, on average, to be larger by 0.2
M_sun at low Z in order to obtain a Chandrasekhar mass white dwarf. This
metallicity dependences may be responsible for a drop of the Type Ia supernova
rate for low metallicity, and may introduce a Z-dependence in the properties of
supernovae. We also estimate the X-ray luminosities of the computed systems,
and investigate their donor star and orbital properties.
