2 Observations and data reduction

The bulk of our observations were collected during several observing campaigns at the Observatoire de Haute-Provence (OHP). We used the Carelec and Aurélie spectrographs respectively attached to the 1.93 and 1.52 m telescopes at OHP. Table 1 provides a summary of the various spectral ranges covered and the corresponding reciprocal dispersions. For the Carelec observations until 1990, we used a thin back illuminated RCA CCD with 323$\times$512 pixels of 30 $\mu$m squared. In 1995, the detector was a Tektronix TK512 CCD with a pixel size of 27 $\mu$m. Aurélie was equipped with a Thomson TH7832 linear array with a pixel size of 13 $\mu$m until 1999. From 2000 on, the detector of the Aurélie instrument was replaced by a 2048$\times$1024 CCD EEV 42-20#3, with a pixel size of 13.5 $\mu$m squared.

All the data were reduced in the standard way using the IHAP (for data taken before 1995) and MIDAS softwares developed at ESO. We derived a template of the telluric absorption spectrum that affects the spectral ranges centered on H${\alpha }$ and He I $\lambda$ 5876 from observations of a rapidely rotating star obtained at very different airmasses. This template was then used to achieve a first order correction of the telluric features on the spectra of HDE 228766. Finally, the spectra were normalized using properly chosen continuum windows.

Three spectra of HDE 228766 were obtained in October 2001 with the echelle spectrograph at the 2 m telescope of the Observatorio Astronómico Nacional of San Pedro Mártir (SPM). The detector was a Site CCD with 1024$\times$1024 pixels of 24 $\mu$m squared. The slit width was set to 150 $\mu$m corresponding to 2 $^{\prime\prime}$ on the sky. The data were reduced using the MIDAS echelle package and the wavelength-calibrated spectra were rectified using the SPEFO code (Horn et al. 1996).

 

 

Table 1: Overview of our observing campaigns. The second column indicates the instrument used for the observations: A = Aurélie at the OHP 1.52 m, C = Carelec at the OHP 1.93 m, SPM = echelle spectrograph at the SPM 2 m telescope. N yields the number of nights when HDE 228766 was observed.

Campaign	Inst.	Spectral range	Recipr. disp.	N
		(Å)	(Å mm-1)
Aug. 1989	C	6500-10 400	260	1
Sep. 1989	C	8350-11 150	260	1
Aug. 1990	C	6780-7200	33	1
Aug. 1990	C	8300-11 100	260	1
Jul. 1995	C	4530-4720	17	1
Jul. 1999	A	4100-4950	33	7
Aug. 1999	A	4100-4950	33	13
Sep. 2000	A	4455-4905	16	9
Sep. 2000	A	6340-6790	16	9
Sep. 2000	A	5580-6030	16	2
Sep. 2000	A	3980-4440	16	1
Sep. 2001	A	6340-6790	16	7
Oct. 2001	SPM	3700-7370	10	3

Figure 1: Composite spectrum of HDE 228766 from the near ultraviolet to the near infrared. All the observations shown here were taken in September 2000 except for the spectral ranges 7030-7150 and 8600-10 900 that were observed in August 1990. The most prominent stellar and interstellar features are identified. The $\oplus$ symbol indicates spectral regions that are considerably affected by telluric absorptions and for which no first order correction was applied.