  We discuss the dynamics of a Bose-Einstein condensate of atoms which is
suddenly coupled to a condensate of molecules by an optical or magnetic
Feshbach resonance. Three limiting regimes are found and can be understood from
the transient dynamics occuring for each pair of atoms. This transient dynamics
can be summarised into a time-dependent shift and broadening of the molecular
state. A simple Gross-Pitaevskii picture including this shift and broadening is
proposed to describe the system in the three regimes. Finally, we suggest how
to explore these regimes experimentally.
