We present decompositions of azimuthally averaged surface brightness :profi
les in optical B, V, R and I-bands for a sample of 40 M 51-type interacting
galaxies. The profiles were modelled by an exponential disk and a spherica
l bulge described either by the R-1/4 law or by an exponential function. Ha
lf of the galaxies were well fitted by both bulge models, whereas for 35% t
he exponential function was a better choice. Special care was taken on elim
inating superpositions of the companion galaxies. The mean B-band central s
urface brightness mu(0) was found to be 21.5 mag arcsec(-2), which is near
to the value originally found by Freeman (1970), but the scatter was fairly
large. Galaxy interactions have strongly modified the disks in many of the
galaxies in the sample. For example, six of them had extremely flat bright
ness profiles outside the exponential part of the disk, and many showed sig
nificant isophotal twists.