  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 (M0 or later) 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 Msun and orbital
periods of 0.6--1.7 days, with typical uncertainties of ~0.3% in mass and
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 Msun, components in short-period binary systems (P<1
day; 12 stars) have radii which are inflated by up to 10% (mean=4.8+/-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 (mean=1.7+/-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 that the evolutionary models are broadly valid in the M~0.35-0.80 Msun
regime.
