  Classical novae produce radioactive nuclei which are emitters of gamma-rays
in the MeV range. Some examples are the lines at 478 and 1275 keV (from 7Be and
22Na) and the positron-electron annihilation emission, with the 511 keV line
and a continuum. Gamma-ray spectra and light curves are potential unique tools
to trace the corresponding isotopes and to give insights on the properties of
the expanding envelope. Another possible origin of gamma-rays is the
acceleration of particles up to very high energies, so that either neutral
pions or inverse Compton processes produce gamma-rays of energies larger than
100 MeV. MeV photons during nova explosions have not been detected yet,
although several attempts have been made in the last decades; on the other
hand, GeV photons from novae have been detected with the Fermi satellite in
V407 Cyg, a nova in a symbiotic binary, where the companion is a red giant with
a wind, instead of a main sequence star as in the cataclysmic variables hosting
classical novae. Two more novae have been detected recently (summer 2012) by
Fermi, apparently in non symbiotic binaries, thus challenging our understanding
of the emission mechanism. Both scenarios (radioactivities and acceleration) of
gamma-ray production in novae are discussed.
