  Polaronic features similar to those previously observed in the photoinduced
spectra of cuprates have been detected in the reflectivity spectra of
chemically doped parent compounds of high-critical-temperature superconductors,
both $n$-type and $p$-type. In Nd$_2$CuO$_{4-y}$ these features, whose
intensities depend both on doping and temperature, include local vibrational
modes in the far infrared and a broad band centered at $\sim$ 1000 cm$^{-1}$.
The latter band is produced by the overtones of two (or three) local modes and
is well described in terms of a small-polaron model, with a binding energy of
about 500 cm$^{-1}$. Most of the above infrared features are shown to survive
in the metallic phase of Nd$_{2-x}$Ce$_x$Cu0$_{4-y}$, Bi$_2$Sr$_2$CuO$_6$, and
YBa$_2$Cu$_3$O$_{7-y}$, where they appear as extra-Drude peaks. The occurrence
of polarons is attributed to local modes strongly coupled to carriers, as shown
by a comparison with tunneling results.
