Photometric Analysis of Magellanic Cloud R Coronae Borealis Stars in the Recovery Phases of Their Declines*

This paper presents the initial results of a multi-site photometric programme to examine the extraordinary behaviour displayed by 18 R Coronae Borealis (RCB) stars in the Magellanic Clouds (MCs). RCB stars exhibit a unique variability whereby they undergo rapid declines of up to several magnitudes. These are thought to be caused by the formation of dust in the stellar environment which reduces the brightness. The monitoring programme comprised the collection of UBVRI photometric data using five telescopes located at three different southern hemisphere longitudes (Las Campanas Observatory in Chile, Mount John University Observatory in New Zealand, and the Southern African Large Telescope, SALT, in South Africa). Examination of the data acquired in the V and I filters resulted in the identification of a total of 18 RCB declines occurring in four stars. Construction of colour-magnitude diagrams (V versus V - I), during the recovery to maximum light were undertaken in order to study the unique colour behaviour associated with the RCB declines. The combined recovery slope for the four stars was determined to be 3.37 +/- 0.24, which is similar to the value of 3.1 +/- 0.1 calculated for galactic RCB stars (Skuljan et al. 2003). These results may imply that the nature of the dust (i.e. the particle size) is similar in both our Galaxy and the MCs.