WMAP and Planck-LFI total intensity (top row) and polarisation (middle row Stokes Q, bottom row Stokes U) maps at the position of the SNR Cas A. The Q and U polarisation maps show the cloverleaf-shaped pattern typical of beam polarisation. The solid and dashed circles denote the regions we have used for aperture photometry integration and background subtraction to derive the values quoted in Tables A.1 and A.2.