  The high resolution optical spectra of H-deficient stars, R Coronae Borealis
stars and H-deficient carbon stars are analyzed by synthesizing the C2 Swan
bands (0,1), (0,0), and (1,0) using our detailed line-list and Uppsala model
atmosphere, to determine the C-abundances and the 12C/13C ratios which are
potential clues to the formation process of these stars. The C-abundances
derived from C2 bands are about the same for the adopted models constructed
with different carbon abundances over the range 8.5 (C/He = 0.1%) to 10.5 (C/He
= 10%). The carbon abundances derived from C I lines are a factor of four lower
than that adopted for the model atmosphere over the same C/He interval, as
reported by Asplund et al.: 'the carbon problem'. In principle, the carbon
abundances obtained from C2 Swan bands and that adopted for the model
atmosphere can be equated for a particular choice of C/He that varies from star
to star (unlike C I lines). Then, the carbon problem for C2 bands is
eliminated. However, such C/He ratios are in general less than those of the
extreme helium stars, the seemingly natural relatives to the RCB and HdC stars.
The derived carbon abundances and the 12C/13C ratios are discussed in light of
the double degenerate (DD) and the final flash (FF) scenarios. The carbon
abundance and the 12C/13C ratios for the FF product, Sakurai's Object is
derived. The carbon abundance in the Sakurai's object is 10 times higher than
in the RCB star VZ Sgr. On an average, the carbon abundance in the Sakurai's
Object is about 10 to 100 times higher than in RCB stars. The 12C/13C ratio in
Sakurai's Object is 3.4, the equilibrium value, as expected for FF products.
