  The classical nova V2491 Cyg was once suggested to be a recurrent nova. We
have broadly reproduced the light curve of V2491 Cyg by a nova outburst model
on a cold $1.36~M_\odot$ white dwarf (WD), which strongly suggests that V2491
Cyg is a classical nova outbursting on a cold very massive WD rather than a
recurrent nova outbursting on a warmer WD like the recurrent nova RS Oph. In a
long-term evolution of a cataclysmic binary, an accreting WD has been settled
down to a thermal equilibrium state with the balance of gravitational energy
release and neutrino loss. The central temperature of the WD is uniquely
determined by the energy balance. The WD is hot (cold) for a high (low)
mass-accretion rate. We present the central temperatures, ignition masses,
ignition radii, and recurrence periods for various WD masses and mass-accretion
rates. In a classical nova, which corresponds to a low mass-accretion rate, the
WD is cool and strongly degenerated and the ignition mass is large, which
result in a strong nova outburst. In a recurrent nova, the WD is relatively
warmer because of a high mass accretion rate and the outburst is relatively
weaker. The gravitational energy release substantially contributes to the
luminosity during the recurrent nova outbursts. We compare physical properties
between classical novae and recurrent novae and discuss the essential
differences between them.
