The observed R-band surface brightness profiles for M51-type galaxies. If R-profiles were not available, then V- or B-band profiles are shown (indicated in parenthesis). The left and middle panels show the ‘prototypical’ main galaxies and companions and the right panel the pairs with larger galaxy separations. Radius is scaled with the scalelength of the disc and in order better to illustrate the shallow outer profiles; in (b), the fitted exponential profiles were subtracted from the surface brightnesses. So as not to overlap the profiles, they have been shifted vertically.