THE LIGHT AND SPECTRUM VARIATIONS OF VX-SAGITTARII, AN EXTREMELY COOL SUPER-GIANT

Narrow-band photometry covering more than two cycles of its 732-day variation strongly supports the interpretation that VXSgr is an extremely cool supergiant star. The spectral type from near-infrared bands of TiO and VO ranges from M5.5 at maximum light toM9.8 at minimum, while the brightness varies by 1.2mag in 7(104) and by more than 6mag in V in these respects VXSgr resembles a cool Miravariable. Unlike aMira, however, VX Sgr shows strong bands of CN throughout the cycle, corresponding to a la luminosity classification. The TiO band strength is normal for the colour if the interstellar extinction is Av = 1.5 mag; the VO bands, however, are abnormally strong for the colour. Spectral scans at 8 A resolution support the interpretation of VX Sgr as an M-type supergiant by confirming the presence of CN and VO and showing that ZrO is absent. From a rough calibration of the molecular band strengths in terms of temperature and luminosity, we find that the radius varies from 6.3 AU at maximum light to approximately 9 AU at minimum. VX Sgr appears to be a Population I member of the Sagittarius arm at a distance of about 1500 pc. © Royal Astronomical Society.