2 The data

In order to examine the parameters describing the global structures of M 51-type galaxies 21 isolated pairs were observed in B, V, R and I passbands. The pairs were selected on morphological basis, showing tidal tails and bridges and rather regular, deeply penetrating spiral arms in the main galaxy. In most of the selected pairs the companion was considerably fainter than the principal galaxy and its projected position was near the end of the bridge.

The galaxies were imaged with the 1.5 m telescope in Calar Alto, the 2.5 m Nordic Optical and 1 m Jacobus Kapteyn telescopes in La Palma, and with the 2.1 m telescope in San Pedro Martir in Baja California. In most cases the filters were in the Cousins BVRI photometric system, but if r and i were in the Thuan-Gunn system they were transformed to the Cousins system. Standard reduction techniques were used to produce the calibrated images. In the final images global variations in the sky background level were comparable with the background noise, and the magnitudes reached typically the surface brightnesses of $26-27 \ \rm mag \ arcsec^{-2}$ in the B-band. A full description of the observations and data reduction, together with B-V and R-I color maps, can be found in Laurikainen et al. ([1998]) (hereafter Paper I). The analysis of one of the pairs, Arp 86, together with the N-body model has been published separately (Laurikainen et al. [1993]; Salo & Laurikainen [1993]).