Abundance of the α elements Ca, Mg, Si, Ti ii and combined α relative to iron (Fe i) as a function of [Fe i/H]. The sample is split in stars that have spatial and kinematic properties compatible with membership to the Sag stream (green) and not compatible (purple). The small squares indicate the chemical abundances for literature samples of MW halo stars (black ones: Venn et al. 2004; Barklem et al. 2005; Ishigaki et al. 2012, 2013; red and blue: “low-α” and “high-α” populations as identified in Nissen & Schuster 2010, 2011). We warn the reader that the global [α/Fe] shown here and in Figs. 9, 13 has been calculated as the average of the abundance ratio of the individual α-elements to Fe that were available in each of the catalogs.