Implications of a possible 23 day periodicity for binary black hole models in Mkn~501

Abstract: We investigate the implications of a massive binary system in the centre ofthe gamma-ray blazar Mkn~501 and show that the periodical behaviour recentlyobserved in the TeV and X-ray lightcurves may possibly be related to theorbital motion of the relativistic jet emerging from the less massive blackhole. For the special relativistic jet properties inferred from emissionm..