  Two quantum dots with tunable mutual tunnel coupling have been embedded in a
two-terminal Aharonov-Bohm geometry. Aharonov-Bohm oscillations are
investigated in the cotunneling regime. Visibilities of more than 0.8 are
measured indicating that phase-coherent processes are involved in the elastic
and inelastic cotunneling. An oscillation-phase change of pi is detected as a
function of bias voltage at the inelastic cotunneling onset.
