Spectral index maps between 74 MHz and 1.4 GHz (resolution 75′′) for the SNRs in our sample (except the source 3C 397). The maps were constructed by combining the VLSSr image with those available from radio continuum surveys (see text for details). Pixels with brightness below 4σ at 74 MHzor 1.4 GHz were blanked. The colour scales displayed at the top of the maps indicate the spectral indices measured over each SNRs. The radio continuum emission from VLSSr 74 MHz at a resolution of 75′′ is represented by contours. For reference, we used the same contours levels as in Fig. 1.