What Are the R Coronae Borealis Stars? Geoffrey C. Clayton
Department of Physics and Astronomy, Louisiana State University,
Baton Rouge, LA 70803 Invited review paper, received May 7, 2012
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 analo