The carbon Cepheid V553 Cen: evidence of triple-alpha and CNO cycling

We have obtained high-resolution spectroscopic observations of V553 Cen, a short-period type II Cepheid. We have confirmed the findings of previous investigators that V553 Cen is C- and N-rich. For the first time, we show that V553 Cen is also O-poor and Na-rich, and that ^12C/^13C=4+/-1. We do not see s-process element enhancements in this star. When compared to historical observations, our radial velocity measurements indicate that V553 Cen does not have a binary companion. Hence the abundance peculiarities we observe on the surface are due to V553 Cen altering its own composition by nuclear reaction sequences and dredge-up episodes. These results imply that V553 Cen has experienced CN, ON and triple-alpha processing, and has mixed the products of these reactions to its surface. The chemical composition of V553 Cen and the kinematics of the carbon Cepheids indicate that they are of the same population as early R stars. The evolutionary processes that have produced such objects are not known, but are suspected to be due to helium core flashing in low-mass stars of normal metallicity, as suggested by both Lloyd Evans and Dominy.