  We investigate the fundamental breathing mode of a small-sized elongated
Bose-Einstein condensate coupled to a large Fermi sea of atoms. Our
observations show a dramatic shift of the breathing frequency when the mixture
undergoes phase separation at strong interspecies repulsion. We find that the
maximum frequency shift in the full phase-separation limit depends essentially
on the atom number ratio of the components. We interpret the experimental
observations within a model that assumes an adiabatic response of the Fermi
sea, or within another model that considers single fermion trajectories for a
fully phase-separated mixture. These two complementary models capture the
observed features over the full range of interest.
