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tokens: Acknowledgments We thank to the anonymous reviewer for his/her careful reading of the manuscript and for critical and informative comments, which greatly improved and corrected this work. We are grateful to Polish National Science Centre for their support (grant no. 2011/01/D/ST9/00735). The European Very Long Base Interferomery Network (EVN) is a joint facility of European, Chinese, South African and other radio astronomy institutes funded by their national research councils. KG gratefully acknowledges the Poznan Supercomputing and Networking Centre (PCSS, Poland) for computing grant No. 195. This research has made use of the SIMBAD data base operated at Centre de Donnes astronomiques de Strasbourg (CDS), Strasbourg, France.