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tokens: We thank Dr. Jan Robrade of Hamburger Sternwarte for his help in data analysis of the XMM-Newton data. This work was carried out as part of Project A16 funded by the Deutsche Forschungsgemeinschaft (DFG) under SFB 963. AAV acknowledges support from an Ambizione Fellowship of the Swiss National Science Foundation. C.P.F. was supported by the grant ANR 2011 Blanc SIMI5-6 020 01 “Toupies: Towards understanding the spin evolution of stars”. Part of the work was also supported by the COST action MP1104 “Polarisation as a tool to study the Solar-system and beyond”. Finally, we thank the anonymous referee for their detailed and constructive comments.