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We present infrared spectral indices (1.0-2.3 um) of Galactic late-typegiants and red supergiants (RSGs). We used existing and new spectra obtained atresolution power R=2000 with SpeX on the IRTF telescope. While a large COequivalent width (EW), at 2.29 um ([CO, 2.29]>45 AA) is a typical signature ofRSGs later than spectral type M0, [CO] of K-type RSGs and giants are similar.In the [CO, 2.29] versus [Mg I, 1.71] diagram, RSGs of all spectral types canbe distinguished from red giants, because the Mg I line weakens with increasingtemperature and decreasing gravity. We find several lines that vary withluminosity, but not temperature: Si I (1.59 um), Sr (1.033 um), Fe+Cr+Si+CN(1.16 um), Fe+Ti (1.185 um), Fe+Ti (1.196 um), Ti+Ca (1.28 um), and Mn (1.29um). Good markers of CN enhancement are the Fe+Si+CN line at 1.087 um and CNline at 1.093 um. Using these lines, at the resolution of SpeX, it is possibleto separate RSGs and giants. Contaminant O-rich Mira and S-type AGBs arerecognized by strong molecular features due to water vapor features, TiO bandheads, and/or ZrO absorption. Among the 42 candidate RSGs that we observed, allbut one were found to be late-types. 21 have EWs consistent with those of RSGs,16 with those of O-rich Mira AGBs, and one with an S-type AGB. These infraredresults open new, unexplored, potential for searches at low-resolution of RSGsin the highly obscured innermost regions of the Milky Way.

New infrared spectral indices of luminous cold stars: from early K to M-types