Long-term variations in dust production in R Coronae Borealis

Several searches for periodicities in the historical visible light curves of RCB stars have found that hte intervals between declines are random. This paper re-examines 70 declines of R CrB since 1853 using one homogeneous dataset, the AAVSO light curve. In this dataset, pairs of consecutive declines also show a random distribution of intervals. However, if the sequence of declines is examined, there are semi-periodic variations between times of high and low decline activity on a time scale of a few years. Near-IR photometry of RCB stars indicates that there are large semi-periodic variations in the amount of dust being produced which have similar time scales. Possible interpretations of a semi-periodic variation in dust formation rates in RCB stars are: i) a magnetic activity cycle resulting in "spots" on the star over which dust formation takes place. Such a magnetic activity cycle is similar to the solar cycle; ii) changes in the period and amplitude of the pulsations over several years affecting the efficiency of dust production.