We investigate the interband transitions in several ensembles of self-assembled quantum dots by using photoluminescence excitation spectroscopy under strong magnetic fields up to . Well-defined resonances are observed in the spectra. The magnetic field dependence of the resonance energies allows an unambiguous assignment of the interband transitions which involve both discrete states of the quantum dots and wetting layer states. A strong anticrossing between two transitions is observed in all samples, which cannot be accounted for by a purely excitonic model. The coupling between the mixed exciton-longitudinal optical (LO) phonon states is calculated using the Fröhlich Hamiltonian. The excitonic polaron energies as well as the oscillator strengths of the interband transitions are determined. An anticrossing is predicted when two exciton-LO phonon states have close enough energies with phonon occupations which differ by one. A good agreement is found between the calculations and the experimental data evidencing the existence of excitonic polarons.