density are not evident in weaker sources where the signal-to-noise ratio is much smaller. In the case of N-1 the variation is only in position since the flux density remained constant and the flux density variations are evident only for the jet source and S-1. We will discuss our interpretation of the observed variabilities and proper motions in the following subsections.

The central jet source and source S-1
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The jet source seems to have increased its flux density by about 10% between the two epochs (see Table 1 and Figure 1). This difference is consistent with the fact that both thermal jets (Rodríguez et al. 2001; Galván-Madrid, Avila, & Rodríguez 2004) and HH knots (Rodríguez et al. 2000) can show small but statistically significant flux density variations on scales of years or even months. The difference image (Figure 4) suggests that the increase in emission comes from two discrete positions. One increase (of about 0.8 mJy) is unresolved and associated with the central source and we attribute it to an increase in mass loss at the core of the jet. The second increase (of about 0.4 mJy) comes from an unresolved component clearly displaced to the SE, and located 