The periodicities of R Coronae Borealis stars and their shells

All R Coronae Borealis (RCB) stars vary on a time scale of tens of days, though none are completely regular in their behavior. Multiple periodicities can appear, but their significance is not yet established. The period of RY Sgr has been decreasing by about 0.001 day/cycle over the last 80 years. This rate seems to have been modulated on a 10-20 year time scale. HdC stars also vary but generally with lower amplitudes than the RCB stars. The dust shells of RCB stars vary on a time scale of about 1000 days. Times of bright or, brightening, shells seem in general to be times of increased frequency of obscuration minima. Dust formation may occur quasi-periodically, due to a form of relaxation oscillation, thus explaining the about 1000 day time scale. 32 refs