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 ([Warner 1967]). These may have failed to become RCB stars because their luminosities are too low. They are often referred to as `hydrogen-deficient carbon stars' (HdCs), although this is more by accident than design. The term was originally introduced to represent all of the RCBs, extreme helium stars (see below) and the 'failed' RCBs, since all are hydrogen-deficient and carbon-rich. The HdCs appear to show small-amplitude light variations probably due to pulsation ([Lawson & Cottrell 1997]), but no ejected debris shows up as an infrared excess ([Walker 1985], 1986) or an emission-line region ([Brunner et al. 1998]).