  We describe the discovery of a 0.68+0.52 solar mass eclipsing binary (EB)
with an 8.4-day orbital period, found through a systematic search of ten fields
of the Trans-atlantic Exoplanet Survey (TrES). Such long-period low-mass EBs
constitute critical test cases for resolving the long standing discrepancy
between the theoretical and observational mass-radius relations at the bottom
of the main sequence. It has been suggested that this discrepancy may be
related to strong stellar magnetic fields, which are not properly accounted for
in current theoretical models. All previously well-characterized low-mass main
sequence EBs have periods of a few days or less, and their components are
therefore expected to be rotating rapidly as a result of tidal synchronization,
thus generating strong magnetic fields. In contrast, the binary system
described here has a period that is over three times longer than previously
characterized low-mass main sequence EBs, and its components rotate relatively
slowly. It is therefore expected to have a weaker magnetic field and to better
match the assumptions of theoretical stellar models. Our follow-up observations
of this EB yield preliminary stellar properties that suggest it is indeed
consistent with current models. If further observations confirm a low level of
activity in this system, these determinations would provide support for the
hypothesis that the mass-radius discrepancy is at least partly due to magnetic
activity.
