The Galaxy hosts a variety of other extraordinary stars with little or no surface hydrogen. Most are rare, but nevertheless intriguing. Massive WOLF-RAYET STARS are young (Population I) hydrogen-deficient stars, as are binary systems such as Sgr. Low-mass stars like the RCBs belong to an old population. A group of hydrogen-deficient giants with spectra similar to RCBs show no RCB fading events. These have probably failed to become RCB stars because their luminosities are too low.
As well as RCB stars with F- and G-type spectra, several other stars show RCB type light curves. Two, MVSgr and DYCen, are B-type helium-rich giants and are very similar to a group of about twenty `extreme helium stars', A- and B-type stars with weak or no Balmer lines, strong neutral helium and ionized carbon lines. If they are indeed RCB stars, they are not very active. Their surface carbon abundance is by number, the remainder being helium. A more active B-type helium star, V348Sgr shows strong emission lines at all times, and has a surface carbon abundance of by numbers. This star may be related to about twenty hydrogen-deficient central stars of PLANETARY NEBULAE which have similar carbon abundances and strong emission line spectra. These stars are all overluminous and have strong stellar winds. Carbon-rich knots may form in the winds and give rise to RCB behaviour. Of importance to understanding the RCB fading events, the discovery in 1997 of weak RCB-like fading events in CARBON STARS suggests that the process of carbon condensation in stellar atmospheres is not confined to classical RCB stars.
During the past century, three stars have been observed as being both hot subdwarfs and RCB stars at different times. V605Aql is now the hydrogen-deficient central star of a planetary nebula (Abell58). In 1919 it brightened as a slow nova, and there is a report that its spectrum at one time resembled an RCB star. FGSGE was a FAINT BLUE STAR in 1900. Since then, it has become progressively redder and brighter so that now it has the spectrum of a K-type giant. In 1960 it started to pulsate and in 1992 it started to show RCB type fading events. Its surface composition contains the products of many nuclear reaction processes, and is thought to be hydrogen poor. FGSGE may be a new-born RCB star. Most recently, in 1996, an unremarkable FAINT BLUE STAR suddenly brightened and within three years had become a very cool and luminous carbon star, now known as SAKURAI'S OBJECT or V4334Sgr. The spectacular evolution of these three stars and their similarities with RCB stars may eventually help to explain the origin of RCB stars.