  We have investigated charge transport processes in a superconducting
single-electron transistor (S-SET) fabricated in close proximity to a
two-dimensional electron gas (2DEG) in a GaAs/AlGaAs heterostructure. The
macroscopic bonding pads of the S-SET along with the 2DEG form a microstrip
transmission line. We observe a variety of current-carrying cycles in the S-SET
which we attribute to simultaneous tunneling of Cooper pairs and emission of
photons into the microstrip. We find good agreement between these experimental
results and simulations including both photon emission and photon-assisted
tunneling due to the electromagnetic environment.
