  Supermassive black holes are common in centers of galaxies. Among the active
galaxies, quasars are the most extreme, and their black hole masses range as
high as to $6\cdot 10^{10} M_\odot$. Binary black holes are of special interest
but so far OJ287 is the only confirmed case with known orbital elements. In
OJ287, the binary nature is confirmed by periodic radiation pulses. The period
is twelve years with two pulses per period. The last four pulses have been
correctly predicted with the accuracy of few weeks, the latest in 2007 with the
accuracy of one day. This accuracy is high enough that one may test the higher
order terms in the Post Newtonian approximation to General Relativity. The
precession rate per period is $39^\circ.1 \pm 0^\circ.1$, by far the largest
rate in any known binary, and the $(1.83\pm 0.01)\cdot 10^{10} M_\odot$ primary
is among the dozen biggest black holes known. We will discuss the various Post
Newtonian terms and their effect on the orbit solution.
  The over 100 year data base of optical variations in OJ287 puts limits on
these terms and thus tests the ability of Einstein's General Relativity to
describe, for the first time, dynamic binary black hole spacetime in the strong
field regime. The quadrupole-moment contributions to the equations of motion
allows us to constrain the `no-hair' parameter to be $1.0\:\pm\:0.3$ which
supports the black hole no-hair theorem within the achievable precision.

General Relativity and Quantum Cosmology;Astrophysics of Galaxies