Cataclysmic variables (CV), (also U Geminorum Stars) are a class of binary stars containing a white dwarf and a companion star. The companion star is usually a red dwarf, although in some cases it is another white dwarf or a slightly evolved star (subgiant). Several hundred cataclysmic variables are known. From the observational viewpoint, cataclysmic variables are relatively easy to discover. They are usually quite blue objects, whereas the majority of stars are red. The variability of these systems is often quite rapid and strong. Strong ultraviolet or even X-ray emission and peculiar emission lines are other typical properties. The stars are so close to each other that the gravity of the white dwarf distorts the secondary, and the white dwarf accretes matter from the companion. Therefore, the secondary is often referred to as the donor star. The infalling matter forms in most cases an accretion disc around the white dwarf. Strong UV and X-ray emission is often seen from the accretion disc. The accretion disk may be prone to an instability leading to dwarf nova outbursts, when a portion of the disk material falls onto the white dwarf. During the accretion process, mass is accumulating on the white dwarf surface. Usually the donor star is rich in hydrogen. In many cases, the density and temperature at the bottom of the accumulated hydrogen layer eventually rises high enough to ignite nuclear fusion reactions. The reactions burn the bulk of the hydrogen layer to helium in a short time. This is seen as a nova outburst. The outer parts of the hydrogen layer and some of the fusion products are ejected to interstellar space. If the accretion process continues long enough to bring the white dwarf close to the Chandrasekhar limit, the increasing interior density can ignite runaway carbon fusion and trigger a Type Ia supernova explosion, which completely disrupts the white dwarf. Cataclysmic variables are subdivided into several smaller groups, often named after a bright prototype star characteristic of the class. The classes, which can overlap, include SS Cygni, U Geminorum, Z Camelopardalis, SU Ursae Majoris, AM Herculis, DQ Herculis, VY Sculptoris, AM Canum Venaticorum, and SW Sextantis. In some cases the magnetic field of the white dwarf is strong enough to disrupt the inner accretion disk or even prevent disk formation altogether. Magnetic systems often show strong and variable polarisation in their optical light, and are therefore sometimes called intermediate polars (in case of a partially disrupted disk) or polars (in case of prevented disk formation). As noted earlier, classes of variable stars are customarily named after a well-known prototype star. Intermediate polars and polars are sometimes referred to as DQ Herculis stars and AM Herculis stars, respectively.
