R Coronae Borealis

The prototype of a group of peculiar variable stars characterized by sudden and unpredictable dips in brightness of many magnitudes. R Coronae Borealis (R CrB) is normally magnitude 5.8, but every few years becomes up to nine magnitudes fainter when enshrouded in a dust cloud blown off by a strong stellar "wind". About forty stars of similar type are known, typically supergiant stars of spectral type F or G.