Novae as Gamma-ray sources from radioactivities and particle acceleration

Nova explosions synthesise radioactive nuclei, which are emitters of gamma-rays in the MeV range. Lines at 478 and 1275 keV correspond to ⁷Be and ²²Na decays. In addition, positron-electron annihilation also produces gamma-ray emission, with the 511 keV line and a continuum below it. Gamma-ray spectra and light curves are unique toolsto trace the corresponding isotopes and to give insights on the properties ofthe expanding envelope. Photons in the MeV energy range from nova explosions have not been detected yet, because past and current instruments (e.g., CGRO/Comptel and INTEGRAL/SPI) are not sensitive enough, and only novae closer than about 1 kpc are expected to be detected; there have not been candidates so far.Another type of gamma-ray emission from novae originates in particle acceleration in strong shocks related to mass ejection. Shocks can be either external - between the nova ejecta and the dense wind of the red giant companion in symbiotic recurrent novae - or internal - within the nova ejecta of classical novae. Collisions between accelerated protons produce neutral pions which decay emitting photons with energies larger than 100 MeV (hadronic process); relativistic electrons are responsible for Inverse Compton emission, also at high energies (leptonic process). Fermi/LAT has detected high-energy gamma-rays in several novae, of both types, since its launch in 2008.A review and update of both topics will be presented, together with the implications for the understanding of nova nucleosynthesis (MeV gamma-rays) and shocks during mass ejection (GeV gamma-rays).