New ultra metal-poor stars from SDSS: follow-up GTC medium-resolution spectroscopy D. S. Aguado, C. Allende Prieto, J. I. González Hernández, R. Rebolo and E. Caffau A&A, 604 (2017) A9 Published online: 26 July 2017 DOI: 10.1051/0004-6361/201731320