is increasing only slowly. Gaensler et al. (1997) fitted a model consisting of a thin spherical shell to the $uv$-plane data and showed that, between 1992 and 1995, the average expansion velocity of the remnant was only $2800 \pm 400$ km s$^{-1}$. In contrast, the average expansion speed between 1987 and 1991 was about 35,000 km s$^{-1}$ (Jauncey et al. 1988). Comparison of four images obtained between 1992 and 1995 showed that the brightness of the lobes increased relative to that of the spherical shell and that the asymmetry between the east and west lobes increased markedly over this period (Gaensler et al. 1997). By 1995, the peak brightness of the eastern lobe was 1.8 times that of the western lobe.

The only other young supernova which has been imaged with high resolution at radio wavelengths is SN 1993J in M81. Frequent observations using Very Long Baseline Interferometry (VLBI) techniques (e.g. van Dyk et al. 1994, Marcaide et al. 1997, Bartel et al. 2000) have shown that the radio emission is in the form of an expanding shell, the outline of which is nearly circular, but rather lumpy. Unlike SNR 1987A, SN 1993J remains in the radio supernova stage with 