2 Observations and data reduction

Interstellar absorption lines which arise from the (2,0) and (3,0) bands of the C₂ A $^1\Pi_{\rm u}$ - X $^1\Sigma_{\rm g}^+$ Phillips System, around 8765 Å and 7720 $\rm\AA$, respectively, and of interstellar CN in the (2,0) and (1,0) bands of the CN A $^2\Pi _{\rm u}$ - X ${^2\Sigma ^+}$ red system, around 7900 Å and 9150 $\rm\AA$, respectively, were searched towards stars Nos. 5, 7, 8A, 9, 11, and 12, of the Cyg OB2 association. The observations were carried out during eight nights in September 1999, using the fibre-fed echelle spectrograph FOCES of the Calar Alto Observatory 2.2 m telescope. The instrument is described in detail by Pfeiffer et al. (1998). Total integration times were two hours each for Nos. 7, 8A, 9, 11, five hours for No. 5, and twelve hours for No. 12. The 2 pixel resolution of FOCES is $R = \lambda / \Delta \lambda = 65~000$ using a 15 $\mu$m detector. The achieved resolution is about R=45 000, as judged from the width of the thorium lines. The full optical wavelength region of 4000-10 000 Å is obtained with a single integration. The data were reduced using the MIDAS echelle context. The background, including the bias and the scattered light contamination, was defined by fitting a two-dimensional surface to the regions between individual orders. Pixel-to-pixel variations of the detector were corrected by use of integrations on an internal halogen lamp. The location of the spectral orders was determined using the Hough algorithm. Extracted spectra were transformed from pixel to wavelength scale using integrations on internal thorium arcs taken at the beginning and at the end of each night. The spectra were re-binned to a linear, heliocentric wavelength scale. Heliocentric velocities were inferred from the rest wavelengths of Chauville et al. (1977). Uncertainties in the velocities are estimated to be of the order of 1-1.5 km s^-1.