  Recurrent novae are binaries harboring a very massive white dwarf (WD), as
massive as the Chandrasekhar mass, because of their short recurrence periods of
nova outbursts of 10-100 years. Thus, recurrent novae are considered as
candidates of progenitors of Type Ia supernovae (SNe Ia). In fact, the SN Ia
PTF11kx showed evidence that its progenitor is a symbiotic recurrent nova. The
binary parameters of recurrent novae have been well determined, especially for
the ones with frequent outbursts, U Sco and RS Oph, which provide useful
information on the elementary processes in binary evolution toward SNe Ia.
Therefore we use them as testbeds for binary evolution models. For example, the
original double degenerate (DD) scenario cannot reproduce RS Oph type recurrent
novae, whereas the new single degenerate (SD) scenario proposed by Hachisu et
al. (1999) naturally can. We review main differences between the SD and DD
scenarios, especially for their basic processes of binary evolution. We also
discuss observational support for each physical process. The original DD
scenario is based on the physics in 1980s, whereas the SD scenario on more
recent physics including the new opacity, mass-growth efficiency of WDs, and
optically thick winds developed in nova outbursts.
