Diagram of targeted stars in the MSX A–C versus D–E colors. Upper limits and blends have been removed. Targets are marked with blue filled circles (detections) and empty circles (non-detections). As illustration we plot also the positions of astrophysically related objects. Triangles indicate the location of the PNe and diamond symbols that of post-AGB (transitional) objects provided by Ortiz et al. (2005). The long-dashed line marks the locus of a blackbody (F_ν) with a temperature from 2200 K to 200 K.