  The light curves of type-II supernovae (SNe) are believed to be highly
affected by recombination of hydrogen that takes place in their envelopes. In
this work, we analytically investigate the transition from a fully ionized
envelope to a partially recombined one and its effects on the SN light curve.
The motivation is to establish the underlying processes that dominate the
evolution at late times when recombination takes place in the envelope, yet
early enough so that $^{56}$Ni decay is a negligible source of energy. We
consider the diffusion of photons through the envelope while analyzing the
ionization fraction and the coupling between radiation and gas, and find that
the main effect of recombination is on the evolution of the observed
temperature. Before recombination the temperature decreases relatively fast,
while after recombination starts it significantly reduces the rate at which the
observed temperature drops with time. This behaviour is the main cause for the
observed flattening in the optical bands, where for a typical red supergiant
explosion, the recombination wave affects the bolometric luminosity only mildly
during most of the photospheric phase. Moreover, the plateau phase observed in
some type-II SNe is not a generic result of recombination, and it also depends
on the density structure of the progenitor. This is one possible explanation to
the different light curve decay rates observed in type II (P and L) SNe.
