  Reflectance measurements in properly selected samples of
Nd$_{1.96}$Ce$_{0.04}$CuO$_{4+y}$ with different carrier concentrations provide
a firm evidence of Fano antiresonances for the four transverse optical $E_u$
phonons in the $a-b$ plane. Fits performed without any previous subtraction of
the electronic background, as well as the dependence on temperature of the
renormalized phonon frequencies, show that the electronic continuum repsonsible
for the Fano effect is the polaron band found in all parent compounds of
high-T$_c$ superconductors.

Superconductivity