3 Optical spectroscopic observations and data reduction

All sources were investigated during several observing runs performed at Xinglong station, Beijing Astronomical Observatory (BAO) from November 1996 through September 1998. The low resolution spectra were taken with the BAO 2.16 m telescope and the OMR spectrograph, using a Tektronix $1024 \times 1024$ CCD as detector. Two gratings of 300  g  mm^-1 and 150  g  mm^-1 were employed in order to get large wavelength coverage. All observations were made through a 2.3'' slit which produced a resolution of 10 to 11 Å and 20 to 22 Å  as measured from the comparison spectra. Exposure time was generally between 1200 s and 3600 s depending on the brightness of the object. For a few objects, observations were performed twice in order to get a higher S/N ratio spectrum. All observations were obtained at airmass less than 1.5.

The raw two-dimensional data were reduced following standard procedures using the IRAF program package. The CCD reductions included bias subtraction, flatfield correction and cosmic-ray removal. Wavelength calibration was carried out using helium-neon-argon lamps taken at the beginning and end of the night. The flux calibration was derived with 2 to 3 observations of KPNO standard stars (Massey & Strobel 1988) per night. The atmospheric extinction was corrected by using the mean extinction coefficients for Xinglong Station, which were measured by Beijing- Arizona-Taiwan-Connecticut (BATC) multi-color survey (Yan 1995). The telluric O₂ absorption bands at 6870 Å and 7620 Å were not removed. The final calibrated spectra are shown in Fig. 2. Some of the spectra were taken under nonphotometric conditions so that the flux density scale given in the figures should be used only as a rough guide to the brightness of the sources.

Figure 2: Optical spectra of the counterparts to the X-ray sources. $f_\lambda$ in units of 10-15 erg cm-2 s-1 Å-1 is plotted against wavelength in Å

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Table 1: X-ray parameters for four BL Lac candidates

ROSAT name	z	CR	$\Gamma_{{\rm Gal}}^{\dag }$	$f_{\rm X}$	$L_{\rm X}$	$f_{\rm r}$	$\alpha_{{\rm ox}}$	$\alpha_{{\rm ro}}$
1RXS J				e-12	erg s-1	mJy
073720.4+351752	0.213	0.090	1.46	1.69	3.64 1045	27.2	1.19	0.43
082814.5+415359	0.223	0.089	1.60	1.69	4.13 1044	91.0	1.19	0.52
084829.3+811145	0.175	0.118	3.38	3.42	6.67 1044	12.9	1.11	0.36
221944.6+212054	0.20	0.080	2.65	2.83	6.64 1044	177.6	1.43	0.43