We have found that at least seven hydrogen-deficient carbon ( HdC ) and R Coronae Borealis ( RCB ) stars , have ^ { 16 } O/ ^ { 18 } O ratios close to and in some cases less than unity , values that are orders of magnitude lower than measured in other stars ( the Solar value is 500 ) .
Greatly enhanced ^ { 18 } O is evident in every HdC and RCB we have measured that is cool enough to have detectable CO bands .
The three HdC stars measured have ^ { 16 } O/ ^ { 18 } O < 1 , lower values than any of the RCB stars .
These discoveries are important clues in determining the evolutionary pathways of HdC and RCB stars , for which two models have been proposed : the double degenerate ( white dwarf ( WD ) merger ) , and the final helium-shell flash ( FF ) .
No overproduction of ^ { 18 } O is expected in the FF scenario .
We have quantitatively explored the idea that HdC and RCB stars originate in the mergers of CO- and He-WDs .
The merger process is estimated to take only a few days , with accretion rates of 150 { M } _ { \odot } ~ { } yr ^ { -1 } producing temperatures at the base of the accreted envelope of 1.2 - 1.9 \times 10 ^ { 8 } \mathrm { K } .
Analysis of a simplified one-zone calculation shows that nucleosynthesis in the dynamically accreting material may provide a suitable environment for a significant production of ^ { 18 } O , leading to very low values of ^ { 16 } O/ ^ { 18 } O , similar to those observed .
We also find qualitative agreement with observed values of ^ { 12 } C/ ^ { 13 } C and with the CNO elemental ratios .
H-admixture during the accretion process from the small H-rich C/O WD envelope may play an important role in producing the observed abundances .
Overall our analysis shows that WD mergers may very well be the progenitors of O ^ { 18 } -rich RCB and HdC stars , and that more detailed simulations and modeling are justified .