2 Observations and data reduction

The infrared spectroscopic observations reported in this paper were obtained at the ESO-Paranal Observatory with VLT-Antu using the ISAAC spectrograph in service mode. Spectra were taken, under photometric conditions, during the night of May 20, 2000 (UT). The exposure times were 120, 240 and 480 s for the J-band, H-band and K-band spectra, respectively. Sufficient overlapping was assured for composing a single spectrum ranging from 1.09 to 2.57 $\mu$m with a FWHM resolution of 12 (J) to 27 (K) Å. The data were reduced using IRAF by first applying the combined dark and flatfield images as supplied by the ESO service mode operation group. Median sky frames were then combined for each object and spectral window. This process made use of jittered images where the object is located at different positions along the slit. Wavelength calibration was achieved by measuring the location of atmospheric OH emission lines (Rousselot et al. 2000) in the sky background. The flux calibration was performed using observations of the A0 standard HD 216009, made with the same instrumental setup (but with a wider slit) by the ESO operation team as part of the service mode program. Finally, the telluric absorption features were corrected with the aid of the absorption template constructed by dividing the spectrum of HD 216009 with a low order polynomial fit, excluding a few stellar absorption lines. The IRAF task "telluric" was used to find the best scale and shift factors which, when applied to the normalized telluric template, provided a reasonable correction of the telluric absorptions in HD 216009 and science exposures. This procedure worked well, except for the regions between 1.35-1.44 microns and 1.80-1.94 microns, characterized by heavy telluric absorption. These regions, corresponding to the ends of the J, H and K spectra, were excluded from the analysis and are not shown in this paper. Synthetic J, H, and K photometry of our calibrated standard star observations were compared to broadband photometry by Carter & Meadows (1995) showing differences below 0.08 mag.