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Dwarf galaxies provide insights on the processes of star formation andchemical enrichment at the low end of the galaxy mass function, as well as onthe clustering of dark matter on small scales. In studies of Local Group dwarfgalaxies, spectroscopic samples of individual stars are used to derive theinternal kinematics and abundance properties of these galaxies. It is thereforeimportant to clean these samples from Milky Way stars, not related to the dwarfgalaxy, since they can contaminate the analysis of the properties of theseobjects. Here we introduce a new diagnostic for separating Milky Waycontaminant stars -- that mainly constitute of dwarf stars -- and red giantbranch stars targeted in dwarf galaxies. As discriminator we use the trends inthe equivalent width of the nIR MgI line at 8806.8 \AA\ as a function of theequivalent width of CaII triplet lines. This method is particularly useful forworks dealing with multi-object intermediate resolution spectroscopy focusingin the region of the nIR CaII triplet. We use synthetic spectra to explore howthe equivalent width of these lines changes for stars with different properties(gravity, effective temperature, metallicity) and find that a discriminationamong giants above the horizontal branch and dwarfs can be made with thismethod at [Fe/H]> -2 dex. For -2 $\le$ [Fe/H] $\le$ -1, this method is alsovalid to discriminate dwarfs and giants down to approximately one magnitudebelow the horizontal branch. Using a foreground model we make predictions onthe use of this new discrimination method for nearby dwarf spheroidal galaxies,including the ultra-faints. We subsequently use VLT/FLAMES data for theSextans, Sculptor and Fornax dSphs to verify the predicted theoretical trends.

Cleaning spectroscopic samples of stars in nearby dwarf galaxies: The use of the nIR MgI line to weed out Milky Way contaminants