VLBI observations of SNR in M82 - Is 41.95+575 an anomalous SNR or something else ?

We have used the Global and European VLBI Networks at 18cm to study the more compact radio sources in the starburst galaxy M82 with an angular resolution of between 3 and 15mas, corresponding to scales of 0.04 to 0.2pc at the distance of M82. The observations reveal shells ranging in diameter from 40 to 90mas (0.6 to 1.4pc). We have found clear evidence for expansion in a number of these sources and one of the shell remnants (43.31+592) has been found to increase its diameter between 1986 and 1997 by 13.6+/-2mas, corresponding to an average expansion velocity of 9850+/-1500kms^-1. If we assume that the remnant is in free expansion, this is consistent with a supernova event in the early 1960s. Hence this remnant is almost certainly younger than the strongest, most compact source (41.95+575) which was known to be present in the 1960s. 41.95+575 shows no clear evidence for expansion between 1990 and 1997 (-1), consistent with a greater age and does not show a simple shell type structure. 41.95+575 appears to be anomalous and differs from the vast majority of the 50 compact remnants found in M82 which, from MERLIN+VLA imaging, appear to be more conventional shells or disrupted shells.