2 Observations and data reduction

CCD spectra were obtained with the 1.93 m telescope of the Haute Provence Observatory and the 2.16 m telescope in the XingLong station of the Beijing Observatory. A journal of observations is listed in Table 1; the dispersion and the resolution obtained from the FWHM of the lines of the calibration lamp are also given in addition. The data were reduced by the IRAF software mounted on a SUN station. The standard spectral reduction procedure was used. The main parameters involved are indicated later when needed. No correction for atmospheric extinction and no flux calibration were made.

When the wavelength is less than 8400 Å,  the atmospheric absorption lines are strong and must be eliminated. In order to do this each spectrum was divided by an atmospheric spectrum. The sky absorption spectrum was taken as equal to $F=(F_{\rm s}-1)\frac{AM}{AM_{\rm s}}+1$. Where $F_{\rm s}$ is the normalized sky absorption spectrum obtained from the standard early type stars. AM and $AM_{\rm s}$ are the air mass of the star and standard star respectively. Because of variations of water vapor density in the earth's atmosphere (i.e. $\frac{AM}{AM_{\rm s}}$ is variable), we take a $\frac{AM}{AM_{\rm s}}$ value near 1 and make several attempts for each spectrum observed until the resulting spectrum is satisfactory.

   

Table 1: The observing journal and technical parameters

Observing run	Date	Telescope	Band	Dispersion	Resolution
1	1992.6.12-6.15	1.93 m	8100-8630 Å	33 Å/mm	2.0 Å
2	1993.5.30-6.1	1.93 m	8100-8740 Å	33 Å/mm	2.0 Å
3	1996.5.29-5.30	2.16 m	8380-9400 Å	50 Å/mm	3.5 Å
4	1997.7.13	2.16 m	7900-9000 Å	50 Å/mm	3.5 Å
5	1997.11.7-11.8	2.16 m	8000-9000 Å	50 Å/mm	3.5 Å