  We explore a simple theoretical model to describe the properties of Bose
condensed para-excitons in Cu2O. Taking into account the exciton-phonon
interaction and introducing a coherent phonon part of the moving condensate, we
derive the dynamic equations for the exciton-phonon condensate. Within the Bose
approximation for excitons, we discuss the conditions for the moving
inhomogeneous condensate to appear in the crystal. We calculate the condensate
wave function and energy and a collective excitation spectrum in the
semiclassical approximation. The stability conditions of the moving condensate
are analyzed by use of Landau arguments, and two critical velocities appear in
the theory. Finally, we apply our model to describe the recently observed
interference between two coherent exciton-phonon packets in Cu2O.
