decreasing flux density, making further imaging difficult. Recent observations of SN 1980K (Montes et al. 1998) and SN 1979C (Montes et al. 2000) have shown variations in the rate of decline of flux density or, in the case of SN 1979C, possibly small increases, indicating variations in the density of the circumstellar medium, but, like SN 1993J, these objects are best considered to be still in their radio supernova stage.

The slow expansion velocity of SNR 1987A suggests that the expanding shock and the leading ejecta have encountered a significant density enhancement, greatly reducing their velocity (Chevalier 1992; Duffy, Ball & Kirk 1995; Chevalier & Dwarkadas 1995). These models assume spherical symmetry and hence do not account for the increasing asymmetry of the source. Furthermore, they do not predict the continuing increase in brightness of the remnant as observed by Ball et al. (1995) and discussed further below. An alternative explanation for the slow apparent expansion velocity is that the shock excites slowly moving clumps of circumstellar material and then moves on. Ball & Kirk (1992) modelled the emission observed up to day 1800 by shock heating of two clumps and obtained a good fit to the data up to 