  We present a new method to fit the variations of both coordinates of a VLBI
component as a function of time, assuming that the nucleus of the radio source
contains a binary black hole system (BBH system). The presence of a BBH system
produces 2 perturbations of the trajectory of the ejected VLBI components. By
using only the VLBI coordinates, the problem we have to solve reduces to an
astrometric problem. Knowledge of the variations of the VLBI coordinates as a
function of time contains the kinematical information, thus we are able to
deduce the inclination angle of the source and the bulk Lorentz factor of the
ejected component. Generally, there is a family of the BBH system producing the
same fit to our data. To illustrate this method, we apply it to the source
1807+784. We find that the inclination of the source is i = 5.8+-1.8 degrees
and the VLBI component is ejected with a bulk Lorentz factor of 3.7+-0.3. We
determine the family of the BBH system which provides the best fit, assuming at
first that the masses of the 2 black holes are equal and then that the masses
are different. Each family of BBH systems is characterized by Tp/Tb~1.967,
where Tp and Tb are the precession period of the accretion disk of the black
hole ejecting the VLBI component and the orbiting period of the BBH system.
