The Absolute Parameters for NSVS 11868841 and the Oversized Stars in the Low-Mass Eclipsing Binaries

Spectroscopic observations of the low-mass eclipsing binary NSVS 11868841 have been obtained and the radial velocities were derived for both components. The masses and radii determined for the components are $M_1$ =0.870$\pm$0.074\Msun, $M_2$=0.607$\pm$0.053\Msun and $R_1$ =0.983 $\pm$0.030 \Rsun, $R_2$ =0.901$\pm$0.026 \Rsun. Both the primary and secondary stars' radii are 10 \% and 57\% larger than those of zero-age-main-sequence stars with the same masses. This discrepancy may be arisen from the large spot coverage of both stars. We collected absolute parameters of 21 low mass double-lined eclipsing binaries and compared their positions in the mass-radius and mass-effective temperature panels. The large radii and lower effective temperatures are solved neither with difference in metallicity nor in mixing length parameters. These discrepancies in the low mass stars may be originated by magnetic fields causing inhibition convective energy transport which leads to large magnetic spot coverage on the surface of a low mass star.