  A selection of ultracool dwarfs are known to be radio active, with both
gyrosynchrotron emission and the electron cyclotron maser instability being
given as likely emission mechanisms. To explore whether ultracool dwarfs
previously undetected at 8.5 GHz may be detectable at a lower frequency. We
select a sample of fast rotating ultracool dwarfs with no detectable radio
activity at 8.5 GHz, observing each of them at 4.9 GHz. From the 8 dwarfs in
our sample, we detect emission from 2MASS J07464256+2000321, with a mean flux
level of 286 $\pm$ 24 $\mu Jy$. The light-curve of 2MASS J07464256+2000321, is
dominated towards the end of the observation by a very bright, $\approx $100 %
left circularly polarized burst during which the flux reached 2.4 mJy. The
burst was preceded by a raise in the level of activity, with the average flux
being $\approx$ 160 $\mu Jy$ in the first hour of observation rising to
$\approx$ 400 $\mu Jy$ in the 40 minutes before the burst. During both periods,
there is significant variability. The detection of 100% circular polarization
in the emission at 4.9 GHz points towards the electron cyclotron maser as the
emission mechanism. However, the observations at 4.9 GHz and 8.5 GHz were not
simultaneous, thus the actual fraction of dwarfs capable of producing radio
emission, as well as the fraction of those that show periodic pulsations is
still unclear, as indeed are the relative roles played by the electron
cyclotron maser instability versus gyrosynchrotron emission, therefore we
cannot assert if the previous non-detection at 8.5 GHz was due to a cut-off in
emission between 4.9 and 8.4 GHz, or due to long term variability.
