  The R Coronae Borealis (RCB) stars are rare hydrogen-deficient, carbon-rich,
supergiants, best known for their spectacular declines in brightness at
irregular intervals. Efforts to discover more RCB stars have more than doubled
the number known in the last few years and they appear to be members of an old,
bulge population. Two evolutionary scenarios have been suggested for producing
an RCB star, a double degenerate merger of two white dwarfs, or a final helium
shell flash in a planetary nebula central star. The evidence pointing toward
one or the other is somewhat contradictory, but the discovery that RCB stars
have large amounts of 18O has tilted the scales towards the merger scenario. If
the RCB stars are the product of white dwarf mergers, this would be a very
exciting result since RCB stars would then be low-mass analogs of type Ia
supernovae. The predicted number of RCB stars in the Galaxy is consistent with
the predicted number of He/CO WD mergers. But, so far, only about 65 of the
predicted 5000 RCB stars in the Galaxy have been discovered. The mystery has
yet to be solved.
