The Mysterious R Stars

The R stars are a rare class of K-type giant carbon stars. Canonical stellar evolutionary theory cannot explain their existence, yet they have been observed for more than a century. The early-R stars, the warmest in the R class, are enhanced in ¹²C, ¹³C and ¹⁴N relative to the Sun, but not in s-processes elements or oxygen, and are all single stars. We test the idea that binary mergers lead to the formation of the early-R stars by a comparison of binary population synthesis model results with observations.