White dwarfs are the last stages of stellar evolution of stars with masses smaller than 10 Msun approximately. When isolated they cool-down to invisibility, but when they are in close binary systems they can be "rejuvenated" by mass accretion from their companion star, and eventually explode as novae or thermonuclear supernovae. A nova explosion is a hydrogen thermonuclear runaway on top of the white dwarf, made of either carbon and oxygen (CO) or oxygen and neon (ONe). Mass is ejected at high velocities, but the white dwarf is not disrupted and a new explosion is expected to occur after thousands (103-105) of years.

Relatively recently, in 2007, it was predicted by this group for the first time that novae can be particle accelerators (for RS Oph in its 2006 explosion), and thus play a role on the origin of cosmic rays; accelerated protons and electrons will subsequently emit high-energy gamma rays (with energies larger than 100 MeV), through inverse Compton and/or pion decay processes. The Fermi satellite, launched in 2008, has in fact already detected several novae at such energies, confirming our theoretical expectations.