R Corona Borealis stars
Only around 50 known in the Galaxy and Magellanic Clouds. They are rare type of evolved carbon-rich supergiants that are thought to result from the merger of two white dwarfs, called the Double Degenerate (DD) scenario. This scenario is also studied in context of SN Ia explosions, therefore better understanding of RCrB stars is crucial.
- sudden and non-periodic drops in magnitude
- drops by up to 9 magnitudes
- some small variation in the baseline
- the "dark" stage can last from days to years
- recovers slowly to the baseline
RCrB can be distinguished from other Red Giants based on the fact that an RCB’s circumstellar shell seem thinner than that of most common AGB stars Tisserand et al. 2009.
These are another Carbon stars. Their drops in magnitude are much smaller than in RCrBs and seem to be periodical (P~1000d).