  Among the major uncertainties involved in the Chandrasekhar mass models for
Type Ia supernovae are the companion star of the accreting white dwarf (or the
accretion rate that determines the carbon ignition density) and the flame speed
after ignition. We present nucleosynthesis results from relatively slow
deflagration (1.5 - 3 % of the sound speed) to constrain the rate of accretion
from the companion star. Because of electron capture, a significant amount of
neutron-rich species such as ^{54}Cr, ^{50}Ti, ^{58}Fe, ^{62}Ni, etc. are
synthesized in the central region. To avoid the too large ratios of
^{54}Cr/^{56}Fe and ^{50}Ti/^{56}Fe, the central density of the white dwarf at
thermonuclear runaway must be as low as \ltsim 2 \e9 \gmc. Such a low central
density can be realized by the accretion as fast as $\dot M \gtsim 1 \times
10^{-7} M_\odot yr^{-1}$. These rapidly accreting white dwarfs might correspond
to the super-soft X-ray sources.
