A MASSIVE BINARY BLACK-HOLE IN 1928+738

We apply the binary black hole model to explain the wiggles in the milliarcsec radio jet of the superluminal quasar 1928+738 (4C 73.18) observed with VLBI at 1.3 cm wavelength by Hummel et al. The period and amplitude of the wiggles can be explained as due to the orbital motion of a binary black hole with mass of order 10(8) M., mass ratio larger than 0. 1, and orbital radius approximately 10(16) cm. The jet's inclination to the line of sight should be small confirming the standard interpretation of superluminal motion and one-sidedness as due to relativistic motion in a direction close to the line of sight. The small orbital radius suggests that the binary has been losing a significant amount of orbital energy during the last 10(7) yr, possibly by interaction with the matter which is flowing through the active galactic nucleus. The arcsec-scale radio structure provides additional support for a link between activity and binary evolution. If our interpretation of the mass wiggle in this quasar is correct, then many other quasars may contain massive binary black holes as well.