The fundamental properties of low-mass stars are not as well understood as those of their more massive counterparts .
The best method for constraining these properties , especially masses and radii , is to study eclipsing binary systems , but only a small number of late-type ( \geq M0 ) systems have been identified and well-characterized to date .
We present the discovery and characterization of six new M dwarf eclipsing binary systems .
The twelve stars in these eclipsing systems have masses spanning 0.38-0.59 M _ { \sun } and orbital periods of 0.6–1.7 days , with typical uncertainties of \sim 0.3 % in mass and \sim 0.5-2.0 % in radius .
Combined with six known systems with high-precision measurements , our results reveal an intriguing trend in the low-mass regime .
For stars with M = 0.35-0.80 M _ { \sun } , components in short-period binary systems ( P \lesssim 1 day ; 12 stars ) have radii which are inflated by up to 10 % ( \mu = 4.8 \pm 1.0 \% ) with respect to evolutionary models for low-mass main-sequence stars , whereas components in longer-period systems ( > 1.5 days ; 12 stars ) tend to have smaller radii ( \mu = 1.7 \pm 0.7 \% ) .
This trend supports the hypothesis that short-period systems are inflated by the influence of the close companion , most likely because they are tidally locked into very high rotation speeds that enhance activity and inhibit convection .
In summary , very close binary systems are not representative of typical M dwarfs , but our results for longer-period systems indicate the evolutionary models are broadly valid in the M \sim 0.35-0.80 M _ { \odot } regime .