  We have fabricated and measured superconducting single-electron transistors
with Al leads and Nb islands. At bias voltages below the gap of Nb we observe
clear signatures of resonant tunneling of Cooper pairs, and of Coulomb blockade
of the subgap currents due to linewidth broadening of the energy levels in the
superconducting density of states of Nb. The experimental results are in good
agreement with numerical simulations.
