maps were obtained using the task IMAGR of AIPS with the ROBUST parameter set to 0. The synthesized (FWHM) beam was $1\rlap.{''}18\times0\rlap.{''}60; PA = 8^\circ$. The noise level achieved in the final image was 23 $\mu$Jy beam$^{-1}$.

The 3.6-cm wavelength archive data was taken on 2003 September 25 and 30 (epoch 2003.74) in very similar conditions to those of the new observations and the resulting beam was $1\rlap.{''}17\times0\rlap.{''}64; PA = 11^\circ$. This old data set is described in Rodríguez et al. (2005). These data were cross-calibrated in phase and amplitude using the 2006.42 epoch as the model, which serves to globally minimize the differences in positions and flux densities of the sources. The absolute flux density error is estimated to be 10%.

Results and Discussion
======================

To allow a more reliable comparison between the two epochs, both images were convolved to the same angular resolution ($1\rlap.{''}20\times0\rlap.{''}65; PA = 9^\circ$). To improve the relative gain calibration between the two epochs we examined the relative strengths of the 13 sources we identified in the images. It is clear that the ratios of the flux densities of all the sources except S-1 and N-1 had the same constant value of 0.91$\pm$0.02. Hence, we 