The light and spectrum variations of VX Sgr, an exctremely cool supergiant.

Narrow-band photometry covering more than two cycles of the 732-day variation of the object strongly supports the interpretation that VX Sgr is an extremely cool supergiant. The spectral type from near-IR bands of TiO and VO ranges from M5.5 at maximum light to M9.8 at minimum, while the brightness varies by 1.2 mag in I(104) and by more than 6 mag in V; in these respects VX Sgr resembles a cool Mira variable. Unlike a Mira, however, VX Sgr shows strong bands of CN throughout the cycle, corresponding to an Ia luminosity classification. The TiO band strength is normal for the color if the interstellar extinction is 1.5 mag; the VO bands, however, are abnormally strong for the color.