  Eclipsing binaries offer a unique opportunity to determine basic stellar
properties. With the advent of wide-field camera and all-sky time-domain
surveys, thousands of eclipsing binaries have been charted via light curve
classification, yet their fundamental properties remain unexplored, mainly due
to the extensive efforts needed for spectroscopic follow-ups. In this paper we
present the discovery of a short period (P=0.313 days) double-lined M-dwarf
eclipsing binary, CSSJ114804.3+255132/SDSSJ114804.35+255132.6, by
cross-matching binary light curves from Catalina Sky Surveys and
spectroscopically classified M dwarfs from Sloan Digital Sky Survey. We obtain
follow-up spectra using Gemini telescope, enabling us to determine the mass,
radius, and temperature of the primary and secondary component to be M1 =
0.47\pm0.03(statistic)\pm0.03(systematic) M_sun, M2 =
0.46\pm0.03(statistic)\pm0.03(systematic) M_sun, R1 =
0.52\pm0.08(statistic)\pm0.07(systematic) R_sun, R2 =
0.60\pm0.08(statistic)\pm0.08(systematic) R_sun, T1 = 3560\pm100 K, and T2 =
3040\pm100 K, respectively. The systematic error was estimated using the
difference between eccentric and non-eccentric fit. Our analysis also indicates
that there is definitively 3rd-light contamination (66%) in the CSS photometry.
The secondary star seems inflated, probably due to tidal locking of the close
secondary companion, which is common for very short period binary systems.
Future spectroscopic observations with high resolution will narrow down the
uncertainties of stellar parameters for both component, rendering this system
as a benchmark in studying fundamental properties of M dwarfs.
