  A comprehensive ellipsometric study was performed on Fe$_{1-x}$Co$_{x}$Si
single crystals in the spectral range from 0.01 eV to 6.2 eV. Direct and
indirect band gaps of 73 meV and 10 meV, respectively, were observed in FeSi at
7 K. One of four infrared-active phonons that is energetically close to the
direct absorption edge is coupled both to the electrons and to the low-energy
phonon. This is evident from asymmetry in the phonon line shape and a reduction
of its frequency when the absorption edge shifts across the phonon energy due
to the temperature dependence of the direct band gap. As the temperature
increases, the indirect gap changes sign, which manifests as a transition from
a semiconductor to a semimetal. The corresponding gain of the spectral weight
at low energies was recovered within an energy range of several eV. The present
findings strongly support the model indicating that Fe$_{1-x}$Co$_{x}$Si can be
well described in an itinerant picture, taking into account self-energy
corrections.
