  We study the expansion of an ultracold boson-fermion mixture released from an
elongated magnetic trap, in the context of a recent experiment at LENS (G.
Roati et al., Phys. Rev. Lett. 89, 150403 (2002)). We discuss in some details
the role of the boson-fermion interaction on the evolution of the
radial-to-axial aspect ratio of the condensate, and show that it depends
crucially on the relative dynamics of the condensate and degenerate Fermi gas
in the radial direction, which is characterized by the ratio between the
trapping frequency for fermions and for bosons. Our numerical simulations are
in reasonable agreement with the experiment.
