Novae and accreting white dwarfs as progenitors of Type Ia supernovae

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.