243 Seyfert galaxies of type 1, 2, or 3 are listed in the Catalogue of Quasars and AGNs of Veron and Veron (1989) having mv ≤ 15 and vrad ≤ 20,000 km s-1. Eight of these Seyfert galaxies show two nuclei (IR 0248-11, Mkn 739, Mkn 266, Mkn 463, Mkn 673, Arp 220, NGC 6240, NGC 7593). The separations of the nuclei are 3 – 10 arcsec corresponding to 2 – 6 kpc.

To confirm the double nuclear structure for these disturbed galaxies we determined the internal velocity field to be sure that they are the result of two galaxies in the late stages of merging (Kollatschny et al. 1991).

Making the additional assumption that the Seyfert galaxies Mkn 231 (Kollatschny et al. 1992) and Mkn 273 are galaxies in the final stage of merging, having strong tidal arms but unresolved nuclei, one can estimate that 4 percent of all Seyfert galaxies are in the merging process.

The luminosities of the multiple nuclei Seyfert galaxies are extremely high in comparison to morphologically undisturbed Seyfert galaxies. In Table 1, mean values of the visual and blue luminosities and of the far-infrared and radio (6 cm) luminosities as well as the Ha fluxes are listed for both classes. We have separated Seyfert 1 and Seyfert 2 galaxies.

In all cases the luminosities of double nucleus Seyfert galaxies are higher by a factor of more than two with respect to ‘undisturbed’ Seyfert galaxies — except in the soft X-ray range (ROSAT) and the UV.