Implications of a possible 23 day periodicity for binary black hole   models in Mkn~501
  We investigate the implications of a massive binary system in the centre of the gamma-ray blazar Mkn~501 and show that the periodical behaviour recently observed in the TeV and X-ray lightcurves may possibly be related to the orbital motion of the relativistic jet emerging from the less massive black hole. For the special relativistic jet properties inferred from emission models, we derive an intrinsic orbital period of $(6-14)$ yrs and a centre-of-mass distance of $(2.0-3.5) \times 10^{16}$ cm. If the binary is very close with a separation of the order of that for which gravitational radiation becomes dominant, we find a maximum primary mass of $\sim 10^8 M_{\sun}$ and a corresponding secondary mass in the range of $\sim (4-42)\times 10^6 M_{\sun}$ depending on the intrinsic jet properties. Such values are in line with the black hole masses expected from merger scenarios. 
F. M. Rieger, K. Mannheim