Discovery of blue companions to two southern Cepheids: WW Car and FN Vel

A large number of high-dispersion spectra of classical Cepheids were obtained in the region of the Ca II H+K spectral lines. The analysis of these spectra allowed us to detect the presence of a strong Balmer line, Hɛ, for several Cepheids, interpreted as the signature of a blue companion: the presence of a sufficiently bright blue companion to the Cepheid results in a discernible strengthening of the Ca II H + Hɛ line relative to the Ca II K line. We investigated 103 Cepheids, including those with known hot companions (B5-B6 main-sequence stars) in order to test the method. We could confirm the presence of a companion to WW Car and FN Vel (the existence of the former was only suspected before) and we found that these companions are blue hot stars. The method remains efficient when the orbital velocity changes in a binary system cannot be revealed and other methods of binarity detection are not efficient.