We report the detection of eclipses in GJ 3236 , a bright ( I = 11.6 ) very low mass binary system with an orbital period of 0.77 { days } .
Analysis of light- and radial velocity curves of the system yielded component masses of 0.38 \pm 0.02 and 0.28 \pm 0.02 { M } _ { \odot } .
The central values for the stellar radii are larger than the theoretical models predict for these masses , in agreement with the results for existing eclipsing binaries , although the present 5 \% observational uncertainties limit the significance of the larger radii to approximately 1 \sigma .
Degeneracies in the light curve models resulting from the unknown configuration of surface spots on the components of GJ 3236 currently dominate the uncertainties in the radii , and could be reduced by obtaining precise , multi-band photometry covering the full orbital period .
The system appears to be tidally synchronized and shows signs of high activity levels as expected for such a short orbital period , evidenced by strong H \alpha emission lines in the spectra of both components .
These observations probe an important region of mass-radius parameter space around the predicted transition to fully-convective stellar interiors , where there are a limited number of precise measurements available in the literature .