Binary stars and the UVX in early type galaxies
  We use the Hern\'andez-P\'erez $\&$ Bruzual (2013) HB13 stellar population
synthesis models to study the r\^ole of interacting binary pairs as progenitors
of EHB stars. We assemble a sample of 3417 Early Type Galaxies observed both in
the optical (\textit{SDSS}-DR8) and the UV (\textit{GALEX}-GR6). The galaxies
in our sample can be classified according to their position in the
colour-colour diagram as UV weak or red sequence galaxies ($\sim 48\%$), UV
strong or UVX galaxies ($\sim 9\%$), and recent star forming galaxies ($\sim
43\%$). Analysing this sample using the HB13 models for various choices of
basic model parameters we conclude that: (a) The UV$r$ colours of UV weak and
UV strong galaxies are reproduced by the models as long as the fraction of
binary stars is at least 15$\%.$ (b) Higher metallicity models ($Z = 0.02$ and
$Z = 0.03$) reproduce the colours of UV weak and UV strong galaxies better than
lower $Z$ models. The $Z = 0.03$ model is slightly bluer than the $Z = 0.02$
model in the UV strong region, indicating a weak relationship between UVX and
$Z$. (c) The strength of UVX increases with age in the model population. This
is at variance with the results of other models that include binary stars as
progenitors of EHB stars.
