  Modeling VLBI ejections of nuclei of extragalactic radio sources, indicates
that their nuclei contain a binary black hole system. One can derive the
distance and the positions of the two black holes in the plane of the sky. We
can also use the RMS of the time series of the ICRF2 survey to obtain an
estimate of the structure and the size of the nuclei. We will discuss the
possible problems to link VLBI observations and GAIA optical observations of
radio quasars if they contain a binary black hole system.
