We report here the discovery of an optical flare observed in R band from the red-dwarf eclipsing binary CU Cnc whose component stars are at the upper boundary of full convection ( M _ { 1 } = 0.43 and M _ { 2 } = 0.4 M _ { \odot } , M _ { \odot } is the solar mass ) .
The amplitude of the flare is the largest among those detected in R band ( \sim { 0.52 } mag ) and the duration time is about 73 minutes .
As those observed on the Sun , quasi-periodic oscillations were seen during and after the flare .
Three more R-band flares were found by follow up monitoring .
In total , this binary was monitored photometrically by using R filter for 79.9 hours , which reveals a R-band flare rate about 0.05 flares per hour .
These detections together with other strong chromospheric and coronal activities , i.e. , very strong H _ { \alpha } and H _ { \beta } emission features and an EUV and X-ray source , indicate that it has very strong magnetic activity .
Therefore , the apparent faintness ( \sim 1.4 magnitude in V ) of CU Cnc compared with other single red dwarfs of the same mass can be plausibly explained by the high coverage of the dark spots .