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 CaII H+K spectral lines. The analysis of these spectra
allowed us to detect the presence of a strong Balmer line, H$\epsilon$, 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 CaII H + Hepsilon line relative to the CaII 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.
