Centre of the MKW 4s group, showing NGC 4104 and two of the main galaxies which are on the line-of-sight of the extended stellar envelope. This is a composite CFHT/Megacam image which combines two bands, g and r (see text), with size 16 × 16 arcmin² ( kpc²) and is scaled logarithmically in colour. We can see the distinctive boxiness of the envelope. The very faint shells, appearing like ripples along the major axis of NGC 4104, are not easily seen in this image but will be identified below (Sect. 3.2.2 and Fig. 10).