M_K magnitudes versus periods of the variables in the newly selected Apsis sample (black-filled circles). Encircled in red are the very-probable RSGs, and in green are the targets reported as C-rich or S-type stars in the SIMBAD database. For comparison, diamonds mark the location of well-known RSGs (Chatys et al. 2019; Messineo et al. 2021). The red long-dashed line is the period-luminosity relation from Soraisam et al. (2018) and the dotted line is that of Pierce et al. (2000). The boxes indicate the loci of the giant sequences as described in Riebel et al. (2010).