tokens: We thank the referee for comments that pushed us to improve the analysis of Sect. 3.3. S.C. warmly thanks the financial support by PRIN-INAF2014 (PI: S. Cassisi) and the Economy and Competitiveness Ministry of the Kingdom of Spain (grant AYA2013-42781-P). A.P. acknowledges financial support by PRIN-INAF 2012 The M4 Core Project with Hubble Space Telescope (P.I.: L. Bedin).
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