Next: INTRODUCTION . ... The R Coronae Borealis stars (RCB) stars are one of the longest known classes of variable star. ... This spectacular fading is caused by the formation of sooty dust clouds above the surface of the star. The surfaces of RCB stars are unusually poor in hydrogen, and rich in carbon and nitrogen, which implies that they are the remnants of evolved stars. ... However, many questions remian regarding their evolutionary origin and the physical mechanism of dust formation. ...