We present new broad-band optical images of some merging Seyfert galaxies that were earlier considered to be non-interacting objects. On our deep images obtained at the Russian 6-m telescope we have detected elongated tidal envelopes belonging to satellites debris with a surface R-band brightness about 25-26.5 mag/arcsec^2. These structures are invisible in Sloan Digital Sky Survey (SDSS) pictures because of their photometric limit. We found that 35 per cent of the sample of isolated galaxies has undergone merging during the last 0.5-1 Gyr. Our results suggest that statistic studies based on popular imaging surveys (SDSS or Second Palomar Observatory Sky Survey (POSS-II)) can lead to underestimation of the fraction of minor mergers among galaxies with active nuclei (AGN). This fact impacts on statistics and must be taken into consideration when finding connection between minor/major merging or interactions and nucleus activity.
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