2000MNRAS.311..414W

We have used echelle spectra of resolving power 35000 to derive chemical abundances and the ¹²C/¹³C ratio in the 1.9-d carbon Cepheid RT TrA and the Cepheid U TrA, employed as a comparison star. We confirm that RT TrA is very metal-rich with [Fe/H]=+0.4. In addition, C and N are substantially in excess, and a small deficiency in O is present. We interpret these anomalies as resulting from the appearance on the stellar surface of material enriched in ¹²C by the 3-α process, followed by CNO cycling to convert ¹²C to ¹³C and ¹⁴N. In addition, some ¹⁶O has been processed to ¹⁴N. The partial processing of ¹⁶O to ¹⁴N indicates that substantial ¹⁷O may be present. Proton capture seems to have enhanced ²³Na from the Ne isotopes.