Sum of the equivalent widths of the five luminosity-sensitive spectral features noted in Table 2, both for the MK standards listed in Table 1 and for our targets stars with spectral types M4 or earlier. Squares represent luminosity class Ia standards; triangles, class Iab; open circles, class Ib; crosses, class II; and asterisks, class III. The filled circles correspond to our targets. We consider the four stars with a sum of equivalent widths above 2.5 as bona fide supergiants, as no class III standard populates that region.