The R Coronae Borealis connection
The R Coronae Borealis (RCB) stars are rare hydrogen-deficient carbon-rich supergiants which undergo spectacular declines in brightness of up to 8 magnitudes at irregular intervals as dust forms along the line of sight. Understanding the RCB stars is a key test for any theory which aims to explain hydrogen deficiency in post-Asymptotic Giant Branch (AGB) stars. There are two major evolutionary models for the origin of RCB stars: the Double Degenerate and the Final Helium Shell Flash. In the final flash model, there is a close relationship between RCB stars and Planetary Nebulae (PNe). The connection between RCB stars and PNe has recently become stronger, since the central stars of three old PNe (Sakurai's Object, V605 Aq1 and FG Sge) have had observed outbursts that transformed them from hot evolved central stars into cool giants with the spectral properties of an RCB star.