Long-term evolution of the flux density of Stokes Q and U, for twenty-five, 6 cm 12 h syntheses taken in the period from May 2002 to June 2006. The plotted values are averaged over (mostly) 12 h tracks; the peak values in Q and U often reach values of ±5−7 mJy. The uncertainty on the data points is mostly due to calibration errors and the thermal noise error after 12 h averaging is less than 0.05 mJy. We estimate the uncertainty on Stokes U to typically be 0.1% of the Stokes I flux density or about 0.3 mJy. The uncertainty on Stokes Q is larger, about 0.25−0.5% of Stokes I, and this corresponds to about 0.75−1.5 mJy. These uncertainties are consistent with the short-term variations between the various epochs. Detailed lightcurves are shown in other figures.