  KIC 6131659 is a long-period (17.5 days) eclipsing binary discovered by the
Kepler mission. We analyzed six quarters of Kepler data along with supporting
ground-based photometric and spectroscopic data to obtain accurate values for
the mass and radius of both stars, namely M_1=0.922 +/- 0.007 M_sun, R_1=0.8800
+/- 0.0028 R_sun, and M_2=0.685 +/- 0.005 M_sun, R_2=0.6395 +/- 0.0061 R_sun.
There is a well-known issue with low mass (M <<0.8 M_sun) stars (in cases where
the mass and radius measurement uncertainties are smaller than two or three
percent) where the measured radii are almost always 5 to 15 percent larger than
expected from evolutionary models, i.e. the measured radii are all above the
model isochrones in a mass-radius plane. In contrast, the two stars in KIC
6131659 were found to sit on the same theoretical isochrone in the mass-radius
plane. Until recently, all of the well-studied eclipsing binaries with low-mass
stars had periods less than about three days. The stars in such systems may
have been inflated by high levels of stellar activity induced by tidal effects
in these close binaries. KIC 6131659 shows essentially no evidence of enhanced
stellar activity, and our measurements support the hypothesis that the unusual
mass-radius relationship observed in most low-mass stars is influenced by
strong magnetic activity created by the rapid rotation of the stars in
tidally-locked, short-period systems. Finally, using short cadence data, we
show that KIC 6131657 has one of the smallest measured non-zero eccentricities
of a binary with two main sequence stars, where e cos omega=(4.57
+/-0.02)*10^-5.
