  Massive merging black holes will be the primary sources of powerful
gravitational waves at low frequency, and will permit to test general
relativity with candidate galaxies close to a binary black hole merger. In this
paper we identify the typical mass ratio of the two black holes but then show
that the distance when gravitational radiation becomes the dominant dissipative
effect (over dynamical friction) does not depend on the mass ratio. However the
dynamical evolution in the gravitational wave emission regime does. For the
typical range of mass ratios the final stage of the merger is preceded by a
rapid precession and a subsequent spin-flip of the main black hole. This
already occurs in the inspiral phase, therefore can be described analytically
by post-Newtonian techniques. We then identify the radio galaxies with a
super-disk as those in which the rapidly precessing jet produces effectively a
powerful wind, entraining the environmental gas to produce the appearance of a
thick disk. These specific galaxies are thus candidates for a merger of two
black holes to happen in the astronomically near future.
