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Fig. 2


K-band spectra of the candidate Cygnus OB2 M supergiants obtained with NOTCam at the Nordic Optical Telescope. The appearance of the spectra is dominated by the CO bands longward from 2.293 μm, which reach their maximum strength in red supergiants. The features of NaI (2.207 μm) and CaI (2.263 μm), also prominent in red supergiants, are clearly visible in all the spectra. A technical problem affected the response of the detector during the observations of star 5, therefore the shape of the continuum of this star is not reliably represented in the spectrum.

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