  Exact diagonalization calculations show a continuous transition from
delocalized to small polaron behavior as a function of intersite
electron-lattice coupling. A transition, found previously at Hartree-Fock level
[Yonemitsu et al., Phys. Rev. Lett. {\bf 69}, 965 (1992)], between a magnetic
and a non magnetic state does not subsist when fluctuations are included. Local
phonon modes become softer close to the polaron and by comparison with optical
measurements of doped cuprates we conclude that they are close to the
transition region between polaronic and non-polaronic behavior. The barrier to
adiabatically move a hole vanishes in that region suggesting large mobilities.
