Best result of the 2D fit using GALFIT. Left: original r-band image with the positions of the various shells labelled from 1 to 6, with “s” and “n” referring to the southern and northern shells, respectively. The shells were drawn as circular arcs, symmetric with respect to the centre of NGC 4104. The dashed lines correspond to the shells that we could not measure the magnitude, see Table 7. Middle: best-fit model (with a flat background included), simultaneously taking into account the three brightest galaxies in the group. The isocontours from the original r-band image, corresponding to 24 and 26 mag arcsec⁻² (shown in red and green, respectively), are displayed for reference. Right: residue, that is, the original image minus model, with the positions of the shells superimposed. Left and middle images: the background component is present and the same colour scale is used.