  I review various phenomena associated with mass-accreting white dwarfs (WDs)
in relation to progenitors of type Ia supernovae (SNe Ia). The WD mass can be
estimated from light curve analysis in multiwavelength bands based on the
optically thick wind theory. In the single degenerate scenario of SNe Ia, two
main channels are known, i.e., WD + main sequence (MS) channel and WD + red
giant (RG) channel. In each channel, a typical binary undergoes three
evolutional stages before explosion, i.e., the wind phase, supersoft X-ray
source (SSS) phase, and recurrent nova phase in order of time because the
accretion rate decreases with time as the companion mass decreases. We can
specify some accreting WDs as the corresponding stage of evolution.
Intermittent supersoft X-ray source like RX J0513.9-6951 and V Sge are
corresponding to the wind phase objects. For the SSS phase Cal 87-type objects
correspond to the WD+MS channel. For the WD + RG channel, soft X-ray
observations of early type galaxies gave a statistical evidence of SSS phase
binaries. Recurrent novae of U Sco-type and RS Oph-type correspond to the WD +
MS channel and WD + RG channel, respectively. Majority of recurrent novae host
a very massive WD (~> 1.35 Mo) and often show a plateau phase in optical light
curve correspondingly to the long lasted supersoft X-ray phase: These
properties are indications of increasing WD masses.
