Discovery of Fluorine in Cool Extreme Helium Stars

Neutral fluorine (F I) lines are identified in the optical spectra of cool EHe stars. These are the first identification of F I lines in a star's spectrum, and provide the first measurement of fluorine abundances in EHe stars. The results show that fluorine is overabundant in EHe stars. The overabundance of fluorine provides evidence for the synthesis of fluorine in these stars, that is discussed in the light of asymptotic giant branch (AGB) evolution, and the expectation from accretion of an He white dwarf by a C-O white dwarf.