Structure of the nucleus of 1928+738

Modeling trajectories of VLBI components ejected by the nucleus of 1928+738 shows that the VLBI jet contains three trajectory families, meaning that the VLBI components are ejected from three different origins. The fit of components C1, C6, and C8 indicates that the nucleus of 1928+738 contains two binary black hole systems. The first is associated with the stationary components Cg and CS and is characterized by a radius Rbin,1 ≈ 0.220 mas; both black holes ejected VLBI components almost regularly between 1990 and 2010. The second binary black hole system is not associated with stationary components and is characterized by a radius Rbin,2 ≈ 0.140 mas; it ejected only three VLBI components between 1994 and 1999. The two black hole systems are separated by ≈1.35 mas. We briefly discuss the consequences of the existence of binary black holes systems in radio quasars to make the link between radio quasars and Gaia.