We present VLA observations of RS CVn binaries which indicate that the low-frequency polarization inversion usually ascribed to the quiescent radio emission of these systems is in fact due to the presence, much of the time, of a coherent burst emission. Fig. 1 shows two examples of this coherent emission in the behaviour of the light curves of HR1099 in left and right circular polarization at 1.4 GHz. The emission is highly circularly polarized, with one polarized component steady or slowly varying in a way similar to the higher frequency emission (Fig. 1a), while the other shows rapid fluctuations on timescales of ∼10s and a slower modulation on longer timescales. The similarity of the time profiles in two 50 MHz-wide sidebands 30 MHz apart indicates that the fluctuating component is broadband; moreover, the sense of polarization at 1.4 GHz is opposite to that of the quiescent emission at higher frequencies (attributed to gyrosynchrotron emission), which is exactly the behaviour attributed to the 1.4GHz quiescent emission of these systems by Mutel etal. (1987).

The rapid fluctuations and high polarization are characteristic of a coherent emission process. We argue that a combination of two sources of emission is generally present at 1.4 GHz: a very highly (possibly 100%) polarized coherent component, and a weakly-polarized, steady gyrosynchrotron component. Since the polarization at higher frequencies should be the x-mode of (optically thin) gyrosynchrotron emission, we infer that the low frequency polarization probably represents the o-mode. The observed characteristics of the emission then suggest that the most likely mechanism for the highly-polarized component is plasma emission at the fundamental of the plasma frequency. Such plasma emission is relatively rare on the Sun at 1.4 GHz, but could occur in the hotter coronae of RS CVn systems because the higher temperature reduces the free-free opacity which is thought to absorb 1.4 GHz plasma emission on the Sun.