  Coherent radio emission from stars can be used to constrain fundamental
coronal plasma parameters, such as plasma density and magnetic field strength.
It is also a probe of chromospheric and magnetospheric acceleration mechanisms.
Close stellar binaries, such as RS Canum Venaticorum (RS CVn) systems, are
particularly interesting as their heightened level of chromospheric activity
and possible direct magnetic interaction make them a unique laboratory to study
coronal and magnetospheric acceleration mechanisms. RS CVn binaries are known
to be radio-bright but coherent radio emission has only conclusively been
detected previously in one system. Here, we present a population of 14 coherent
radio emitting RS CVn systems. We identified the population in the ongoing
LOFAR Two Metre Sky Survey as circularly polarised sources at 144MHz that are
astrometrically associated with known RS CVn binaries. We show that the
observed emission is powered by the electron cyclotron maser instability. We
use numerical calculations of the maser's beaming geometry to argue that the
commonly invoked 'loss-cone' maser cannot generate the necessary brightness
temperature in some of our detections and that a more efficient instability,
such as the shell or horseshoe maser, must be invoked. Such maser
configurations are known to operate in planetary magnetospheres. We also
outline two acceleration mechanisms that could produce coherent radio emission,
one where the acceleration occurs in the chromosphere and one where the
acceleration is due to an electrodynamic interaction between the stars. We
propose radio and optical monitoring observations that can differentiate
between these two mechanisms.
