  We present optical spectroscopy and extensive $RI$ differential photometry of
the double-lined eclipsing binary NSVS 02502726 (2MASS J08441103+5423473).
Simultaneous solution of two-band light curves and radial velocities permits
determination of precise emprical masses and radii for both components of the
system. The analysis indicates that the primary and secondary components of
NSVS 02502726 are in a circular orbit with 0.56-day orbital period and have
stellar masses of M$_1$=0.714$\pm$0.019 \Msun, and M$_2$=0.347$\pm$0.012 \Msun.
Both of the components have large radii, being R$_1$=0.67$\pm$0.01 \Rsun, and
R$_2$=0.76$\pm$0.01 \Rsun. The principal parameters of the mass and radius of
the component stars are found with an accuracy of 3% and 1%, respectively. The
secondary component's radius is significantly larger than model predictions for
its mass, similar to what is seen in almost all of the other well-studied
low-mass stars which belong to double-lined eclipsing binaries. Strong
H$_{\alpha}$ emission cores and considerable distortion at out-of-eclipse light
curve in both $R$ and $I$ bandpasses, presumably due to dark spots on both
stars, have been taken as an evidence of strong stellar activity. The distance
to system was calculated as 173$\pm$8 pc from the $BVRIJHK$ magnitudes. The
absolute parameters of the components indicate that both components are close
to the zero-age main-sequence. Comparison with current stellar evolution models
gives an age of 126 $\pm$30 Myr, indicating the stars are in the final stages
of pre-main-sequence contraction.
