Acknowledgements The authors wish to thank two anonymous referees for valuable comments which led to an improved version of the original paper. This work was partially developed while Ana M. Bianco and Graciela Boente were visiting the Departamento de Estatística, Análise Matemática e Optimización de la Universidad de Santiago de Compostela, Spain under the bilateral agreement between the Universidad de Buenos Aires and the Universidad de Santiago de Compostela. This research was partially supported by Grants pict 2014-0351 from anpcyt and 20020130100279BA from the Universidad de Buenos Aires, Argentina and also by the Spanish Projects MTM2013-41383P and MTM2016-76969P from the Ministry of Science and Innovation, Spain. A. Bianco and G. Boente also wish to thank the Minerva Foundation for its support to present some of this paper results at the International Conference on Robust Statistics 2017.