LONG-TERM VARIATIONS IN DUST PRODUCTION IN R-CORONAE-BOREALIS

Several searches for periodicities in the historical visible light cur ves of RCB stars have found that the intervals between declines are ra ndom. This paper reexamines 70 declines of R Coronae Borealis since 18 53 using one homogeneous data set, the AAVSO light curve. In this data set, pairs of consecutive declines also show a random distribution of intervals. However, if the sequence of declines is examined, there ar e semiperiodic variations between times of high- and low-decline activ ity on a time scale of a few years. Near-IR photometry of RCB stars in dicates that there are large semiperiodic variations in the amount of dust being produced which have similar time scales. Possible interpret ations of a semiperiodic variation in dust formation rates in RCB star s are as follows: (i) A magnetic activity cycle resulting in ''spots'' on the star over which dust formation takes place. Such a magnetic ac tivity cycle is similar to the solar cycle; (ii) changes in the period and amplitude of the pulsations over several years affecting the effi ciency of dust production.