2006ApJ...648L.143P

Neutral fluorine (F I) lines are identified in the optical spectra of cool extreme helium (EHe) stars. These are the first F I lines identified in a star's spectrum, and they 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 us with evidence for the synthesis of fluorine in these stars, which is discussed in light of asymptotic giant branch evolution and the expectation from accretion of an He white dwarf by a C-O white dwarf.