these LCs. No long-term trends were found at these wavelengths.

We then compared the results of the correlation and trend analyses to the results obtained for other TeV blazars in . The sources in which no connection between flaring behaviour nor long term behaviour were found were a minority in that sample and were either very weak sources, or bright sources with clear outbursts like S2 0109+22. These other bright sources in the sample were S5 0716+714, ON 325 and W Com and it was suggested that as there were several $2\,\sigma$ peaks in their correlation function, there might be several time-scales involved, blurring the correlation. However, for S2 0109+22 we do not find any correlation peaks above $2\,\sigma$. This result may indicate that a major fraction of the optical flux in this source is not originating from the same emission region as the radio, or that the radio-optical correlation is more complex than can be probed by the simple cross-correlation function used in this paper.

### Optical Polarization

The optical emission in active galaxies is dominated by synchrotron emission of the their jet, which is intrinsically highly polarized. In an optically thin jet with uniform magnetic field, the polarization 