R Coronae Borealis is a low-mass yellow supergiant star in the constellation of Corona Borealis. It is the prototype of the R Cor Bor class of variable stars, which fade by several magnitudes at irregular intervals. R Coronae Borealis itself normally shines at approximately magnitude 6, just about visible to the naked eye, but at intervals of several months to many years fades to as faint as 15th magnitude. Over successive months it then gradually returns to its normal brightness, giving it the nickname "reverse nova", after the more common type of star which rapidly increases in brightness before fading. Nomenclature R Coronae Borealis is a faint naked eye star, but does not have any traditional names. Johann Bayer did not give it a Greek letter designation although it is marked on his map. John Flamsteed numbered all the Bayer stars but did not add any additional designations for fainter stars, so R Coronae Borealis does not appear in either of these two catalogues. At its discovery it was described simply as "the variable in the Northern crown". It was later referred to as Variabilis Coronae, "Variable (star) of Corona (Borealis)". It has also been called a "reverse nova" because of its habit of fading from sight. The variable star designation R Coronae Borealis was introduced, as "Coronae R" by Friedrich Wilhelm Argelander in 1850. Variability The variability of R Coronae Borealis was discovered by English astronomer Edward Pigott in 1795. In 1935 it was the first star shown to have a different chemical composition to the Sun via spectral analysis. R Coronae Borealis is the prototype of the R Coronae Borealis class of variable stars. It is one of only two R Coronae Borealis variables bright enough to be seen with the naked eye, along with RY Sagittarii. Much of the time it shows variations of around a tenth of a magnitude with poorly defined periods that have been reported as 40 and 51 days. These correspond to the first overtone and fundamental radial pulsation modes for an extreme helium star slightly under . At irregular intervals a few years or decades apart R Coronae Borealis fades from its normal brightness near 6th magnitude for a period of months or sometimes years. There is no fixed minimum, but the star can become fainter than 15th magnitude in the visual range. The fading is less pronounced at longer wavelengths. Typically