The metal-insulator transition in manganites is strongly influenced by the
concentration of holes present in the system. Based upon an orbitally degen
erate Mott-Hubbard model, we analyze two possible localization scenarios to
account for this doping dependence: First, we rule out that the transition
is initiated by a disorder-order crossover in the orbital sector, showing
that its effect on charge itineracy is only small. Second, we introduce the
idea of orbital polarons originating from a strong polarization of orbital
s in the vicinity of holes. Considering this direct coupling between charge
and orbital degrees of freedom in addition to lattice effects we are able
to explain well the phase diagram of manganites for low and intermediate ho
le concentrations. [S0163-1829(99)12543-8].