  We have explored the hypothesis that compact subgroups lying within dense
environments as loose groups of galaxies, at a certain stage of their
evolutionary history, could be influenced by the action of the tidal field
induced by the gravitational potential of the whole system. We argue that empty
rings observed in projection around many compact subgroups of galaxies embedded
in larger hosts originate around the spherical surface drawn by the tidal
radius where the internal binding force of the compact subgroup balances the
external tidal force of the whole system. This effect would torn apart member
galaxies situated in this region determining a marked isolation of the
subgroups from the rest of the host groups. If so, subsequent evolution of
these subgroups should not be affected by external influences as the infall of
new surrounding galaxies on them. Following this idea we have developed a
statistical method of investigation and performed an application to show
evidences of such effect studying a loose group of galaxies hosting a compact
group in its central region. The system UZC 578 / HCG 68 seems to be a fair
example of such hypothesized process.
