The MACHO project LMC variable star inventory. X. The R Coronae Borealis stars

We report the discovery of eight new R Coronae Borealis (RCB) stars in the Large Magellanic Cloud (LMC) using the MACHO project photometry database. The discovery of these new stars increases the number of known RCB stars in the LMC to thirteen. We have also discovered four stars similar to the Galactic variable DY Per. These stars decline much more slowly and are cooler than the RCB stars. The absolute luminosities of the Galactic RCB stars are unknown since there is no direct measurement of the distance to any Galactic RCB star. Hence, the importance of the LMC RCB stars. We find a much larger range of absolute magnitudes (M-V = -2.5 to -5 mag) than inferred from the small pre-MACHO sample of LMC RCB stars. It is likely that there is a temperature-M, relationship with the cooler stars being intrinsically fainter. Cool (similar to 5000 K) RCB stars are much more common than previously thought based on the Galactic RCB star sample. Using the fairly complete sample of RCB stars discovered in the MACHO fields, we have estimated the likely number of RCB stars in the Galaxy to be similar to 3200. The SMC MACHO fields were also searched for RCB stars, but none were found.