The R Coronae Borealis stars (RCB) stars fade dramatically and unpredictably by factors of up to one thousand within a few weeks. Over succeeding months, they gradually recover their original brightness. 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. Practically all RCB stars pulsate, which may help to explain the dust formation episodes.