  We experimentally demonstrate and numerically simulate a new adaptive method
to maintain synchronization between coupled nonlinear chaotic oscillators, when
the coupling between the systems is unknown and time-varying (e.g., due to
environmental parameter drift). The technique is applied to optoelectronic
feedback loops exhibiting high dimensional chaotic dynamics. In addition to
keeping the two systems isochronally synchronized in the presence of a priori
unknown time-varying coupling strength, the technique provides an estimate of
the time-varying coupling.
