  Recently, effects of Feshbach resonances in atom-atom interactions were
observed by varying the external magnetic field of an atomic Bose-Einstein
condensate (BEC). We point out that the quasi-bound molecules created in the
intermediate state of the resonance can form a second, molecular condensate.
The many-body state of the system is then a hybrid atomic/molecular condensate
with inter-condensate tunneling of atom pairs. A sudden variation of the
magnetic field results in oscillations of the number of atoms and molecules in
their respective condensates, providing a signature of this novel type of
quantum tunneling.
