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tokens: We acknowledge L. Fuhrmann for careful reading and for useful comments and suggestions to the manuscript. We thank the anonymous referee for comments and criticism that helped to improve this manuscript. C.M.F. was supported for this research through a stipend from the International Max Planck Research School (IMPRS) for Astronomy and Astrophysics at the Universities of Bonn and Cologne. Part of this work was supported by the COST Action MP0905 “Black Holes in a violent Universe”. E.R. acknowledges partial support from MINECO grants AYA2009-13036-C02-02 and AYA2012-38491-C02-01 as well as Generalitat Valenciana grant PROMETEO/2009/104. M.P. acknowledges financial support from MINECO grants AYA2010-21322-C03-01, AYA2010-21097-C03-01, and CONSOLIDER2007-00050. This work is based on observations with the VLBA, which is operated by the NRAO, a facility of the NSF under cooperative agreement by Associated Universities Inc. This research made use of data from the MOJAVE database that is maintained by the MOJAVE team ( Lister et al. 2009 ).