  Single-charge tunneling in ultrasmall voltage biased SIS junctions in a
high-impedance electromagnetic environment is considered. The Cooper pair
current is calculated at T=0 on the basis of the elementary tunnel Hamiltonian
for quasiparticles. Therefore, the transfer of Cooper pairs emerges
automatically as an effect of higher order perturbation theory. The
supercurrent also depends on the dissipative part of the Josephson current
amplitude.
