  We report on the effect of the frequency-locked transfer of single Cooper
pairs in a superconducting single-electron Al transistor embedded in a
dissipative environment (on-chip Cr resistor of R = 40 kOhm). The transistor
was dc voltage biased, and the harmonic signal of frequency f of several MHz
was applied to the gate. Due to the substantial rate of relaxation, the
unidirectional transfer of single pairs occurred in each junction once per
clock cycle and the current plateaus at I = 2ef were developed in the
transistor's I-V curves. The mechanisms (supercurrent, Landau-Zener tunneling,
quasiparticle tunneling, etc.) deteriorating the phase-locking regime are
discussed.
