  The evolution with doping and temperature of the polaronic absorption in a
family of high-T$_c$ superconductors is first followed here across the whole
phase diagram. The polaron band softens with increasing doping, is still
present in the superconducting phase, and vanishes in the overdoped metallic
phase. These results point toward the existence of polaron aggregates at low
temperature and may provide an experimental ground for the increasing number of
theoretical descriptions of many-polaron systems.
