  Based on a sample of 45 main-sequence solar-type spectroscopic binaries we
find the tidal circularization cutoff in the open cluster NGC 188 (age 7 Gyr)
to be well-defined at a period of 15.0 days. When compared to shorter tidal
circularization cutoff periods in the younger clusters M67 and the Hyades, this
result establishes that tidal circularization processes are effective in
solar-type main-sequence binaries for timescales longer than 1 Gyr. This result
is in contrast to the theoretical prediction by Zahn & Bouchet that tidal
circularization due to equilibrium tides is ineffective for late-type main
sequence stars, and that tidal circularization cutoff periods are instead set
only by pre-main-sequence tidal circularization. The NGC 188 cutoff period is
also longer than can be explained by present theories of tidal circularization
due to dynamical tides.
