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F. Comeron (ESO), A. Pasquali (ESO/ST-ECF)
The galactic OB association Cygnus OB2 has been observed at Calar Alto Observatory during the first NEON (Network of European Observatories in the North) Summer School. Known to be the richest association in OB stars in the Local Group (Massey & Thompson 1991) and heavily reddened, Cygnus OB2 has been surveyed at infrared wavelengths in order to unveil its stellar population from a deep sample, complete down to J = 16.5, H = 15.5 and K = 15. A field of 17' by 14' centered on the association centre has been indeed observed in JHK and has allowed us to build the infrared, colour-magnitude diagramme for almost 13070 stars. This is the number of sources detected at least in one filter. We have at the same time obtained IR spectra of the reddest stars in the two-colours diagramme in order to establish their spectral type. The presence of the CO overtone absorptions in all the spectra classifies these objects as background K and M giants. Further calculations point out that OB stars as red as the detected objects could be picked up in our survey; the fact that this has not occurred qualitatively limits the number of forming and young OB stars in the association. In addition, our deep exposures reveal a cluster of highly reddened stars and nebulosity as the near-infrared counterpart of the thermal radiosource ECX 6,lying in the direction of Cygnus OBs and possible physically belonging to the association.