Results. We discovered radio emission at X band spatially coincident with the equal-mass M7.5 binary with a flux density of 60 mu Jy. We determined a spectral index alpha = -1.1 +/- 0.3 between 8 and 12 GHz, suggesting that non-thermal, optically thin, synchrotron, or gyrosynchrotron radiation is responsible for the observed radio emission. Interestingly, no signal is seen at L band where we set a 3 sigma upper limit of 20 mu Jy. This might be explained by strong variability of the binary or self-absorption at this frequency. By adopting the latter scenario and gyrosynchrotron radiation, we constrain the turnover frequency to be in the interval 5-8.5 GHz, from which we infer the presence of kG-intense magnetic fields in the M7.5 binary. Our data impose a 3 alpha upper bound to the radio flux density of the L7 object of 9 mu Jy at 10 GHz.