Context :

Aims : With the purpose to investigate the radio emission of new ultracool objects , we carried out a targeted search in the recently discovered system VHS J125601.92 - 125723.9 ( hereafter VHS 1256 - 1257 ) ; this system is composed by an equal-mass M7.5 binary and a L7 low-mass substellar object located at only 15.8 pc .

Methods : We observed in phase-reference mode the system VHS 1256 - 1257 with the Karl G. Jansky Very Large Array at X - and L - band and with the European VLBI Network at L -band in several epochs during 2015 and 2016 .

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 \pm 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- \sigma upper bound to the radio flux density of the L7 object of 9 \mu Jy at 10 GHz .

Conclusions :