laboratories for studying effects of outer accretion on galaxy structure and evolution.

In conclusion let us pay attention to the fact that both NGC 3808B and NGC 6286 are viewed edge-on. It is such orientation that allows to include these in PRC, based on optical morphology. If NGC 3808B and NGC 6286 were observed at smaller angles to the line of sight, faint diffuse ring structures being projected onto the more bright regions of main galaxies would be indistinguishable and then the galaxies would not be included in PRC as candidates in PRGs. Undoubtedly, among double interacting galaxies other systems must exist which are similar to considered here but viewed by less advantageous angles for discovering the ring structures. For the search of such objects it would be interesting to undertake detail kinematic investigation of the sample of M 51-type galaxies (there are 160 such systems pointed out by Vorontsov-Velyaminov 1975).

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