Infrared photometry of high-luminosity supergiants earlier than M and the interstellar extinction law

Photometric measurements from 2.3 to 23 microns are reported for some luminous F, G, and K supergiants. The infrared excesses observed do not appear to be representative of the class. At about 4.9 microns, and absorption feature with a depth of 0.4 mag appears abruptly in the supergiants near spectral type G5. This feature is possibly due to absorption by the fundamental vibration band of either CO or CN. The color-excess method yields a reddening curve which gives a value for the ratio of total to selective extinction of about 3.40.