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We report the discovery of a uniquely large excess of 18O in thehydrogen-deficient carbon (HdC) star, HD 137613, based on a spectrum of thefirst overtone bands of CO at 2.3-2.4 micron in which three strong absorptionbands of 12C18O are clearly present. Bands of 12C16O also are present but nobands of 13C16O or 12C17O are seen. We estimate an isotopic ratio 16O/18O ~1.The Solar value of this ratio is ~500. Neither He-core burning nor He-shellflash burning can produce the isotopic ratios of oxygen and carbon observed inHD 137613. However, a remarkable similarity exists between the observedabundances and those found in the outer layers of the broad He-shell ofearly-AGB stars, soon after the end of He-core burning. It is not known how theouter envelope down to the He-shell could be lost but some mechanism ofenhanced mass loss must be involved. HD 137613 may be a post-early-AGB starwith the outer layers of the former He-burning shell as its photosphere.  The unusual elemental abundances of the HdC stars resemble those of the RCoronae Borealis (RCB) stars, but HdC stars do not produce clouds of dust thatproduce declines in brightness. None of the other RCB or HdC stars observedshows significant 18O.Comment: 12 pages, 2 figures, ApJ Letters, in pres

An Extremely Large Excess of 18 O in the Hydrogen-deficient Carbon Star HD 137613

An Extremely Large Excess of ¹⁸ O in the Hydrogen-deficient Carbon Star HD 137613