Any star whose light output varies, whether regularly or irregularly. The graph showing brightness as a function of time for a variable is known as the light curve. There are a number of physical reasons why stars vary, and these are used as the basis for broad groupings according to type. Within each group, many more specialized types may be distinguished, often named after a prototype star. Eruptive and cataclysmic variables are characterized by their unpredictability and include a wide variety from T Tauri stars, in the process of formation, to supernovae, which have reached an explosive end. This group also includes flare stars, novae and dwarf novae. Pulsating variables are physically oscillating because of internal instabilities. These include Cepheid variables, RR Lyrae stars and Mira stars. The other major group consists of the eclipsing binaries, in which the light variation is caused by one star periodically passing in front of another. Such binaries are often also interacting. The prime example is Algol. Some stars, BY Draconis stars for example, show variability as they rotate because their surface brightness is not uniform. The method of naming variable stars is somewhat strange. It is mainly due to F. W. A. Argelander 1799-1875, who used the letters R to Z in conjunction with the genitive case of the constellation name for the nine brightest variables in each constellation. After that, pairs of letters, RR to RZ, SS to SZ and so on to ZZ were used J being omitted. For further variables, the pairs of letters AA to AZ, BB to BZ and so on were introduced, bringing the number of available designations to 334. Since many more than 334 variables are now known in many constellations, they are designated as V335, V336, and so on.