R Coronae Borealis

R Coronae Borealis is a fascinating star. Most of the time this ancient Population II star is just on the limit of visibility at 6th magnitude, but out of the blue it will fade down to magnitude 12 - 250 times fainter! When studied at minima, the light curve resembles a "reverse nova," and has a peculiar spectrum. The reason for this is that this star is carbon rich and ejects shells of dark soot that obscure the starlight.