  We observe dynamic nuclear polarization in a GaAs double dot system using two
electrons that are never exchanged with the reservoir. By periodically bringing
the system to the mixing point where the singlet and the triplet T+ states are
degenerate, we observe that an excess polarization is built up. Surprisingly,
the pumping procedure is most effective when the total duty cycle equals a
multiple of the Larmor precession time of the Ga and As nuclei. The induced
polarization corresponds to cooling of the underlying nuclear system. The
dependence on the dwell time at the mixing point is found to be non-monotonic.
