Isotopic ratios in the red giant component of the recurrent nova T Coronae Borealis

We report the determination of abundances and isotopic ratios for C, O, and Si in the photosphere of the red giant (RG) component of the recurrent nova (RN) T Coronae Borealis from new 2.284-2.402 μm and 3.985-4.155 μm spectroscopy. Abundances and isotopic ratios in the photosphere may be affected by (i) processes in the RG interior which are brought to the surface during dredge-up and (ii) contamination of the RG, either during the common envelope phase of the binary evolution or by material synthesized in RN eruptions, or a combination of the two. We find that the abundances of C, O, and Si are reasonably consistent with the expected composition of an RG after first dredge-up, as is the ¹⁶O/¹⁷O ratio. The ²⁸Si/²⁹Si ratio is found to be 8.6 ± 3.0, and that for ²⁸Si/³⁰Si is 21.5 ± 3.0. The ¹²C/¹³C ratio (10 ± 2) is somewhat lower than expected for first dredge-up. The ¹⁶O/¹⁸O ratio (41 ± 3) is highly inconsistent with that expected either from RG evolution (~550) or from contamination of the RG by the products of a nova thermonuclear runaway. In particular, the C and O isotopic ratios taken in combination are a puzzle. We urge confirmation of our results using spectroscopy at high resolution. We also encourage a thorough theoretical study of the effects on the secondary star in an RN system of contamination by ejecta having anomalous abundances and isotopic ratios.