A dwarf nova (DN) is the flash of
a type of cataclysmic variable star (a white dwarfaccreting matter from a binary companion),
that is generally less bright than a (typical) nova.
Current theory ascribes dwarf novas to heating of accretion disk
material due to a short-term increase in the accretion rate,
i.e., some instability in the ongoing process.
The term dwarf nova is also used to refer to the star, which
is also termed a U Geminorum variable (UG).
Some subtypes: