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Reduction of long-slit spectra

Data reduction was carried out using the MIDAS software package (Grosbøl [1989]). Procedures included bias subtraction, cosmic-ray removal and flat-field correction. The flat-field correction was produced with the normalization algorithm described by Shergin et al. ([1996]). After the wavelength mapping and night sky subtraction, each 2D frame was corrected for atmospheric extinction and was flux calibrated. To derive the sensitivity curves, we used the spectral energy distributions of the standard stars from Bohlin ([1996]). Average sensitivity curves for grisms were produced for each observing night. R.m.s. deviations between the average and individual sensitivity curves are $\approx$1.5%, with the maximum deviations of $\approx$4% in the spectral region 3800$\div$4000 Å.

The 2D flux-calibrated spectra were then corrected for atmospheric dispersion (see Kniazev et al. [2000]) and averaged. Finally, the 1D averaged spectrum was extracted from a 08 region along the slit, where > 2$\sigma$ ($\sigma$ is the dispersion of a noise statistics around this line) (see Fig. 2).

Redshift and line fluxes were measured applying Gaussian fitting. For H$\alpha$, [N II] $\lambda\lambda$6548,6583 Å and [S II] $\lambda\lambda$6716,6731 Å a deblending procedure was used, assuming gaussian profiles with the same FWHM as for single lines. The errors of the line intensities in Table 2 take into account the Poisson noise statistics and the noise statistics in the continuum near each line, and include uncertainties of data reduction. These errors have been propagated to calculate element abundances.

The observed emission line intensities $F(\lambda)$, and those corrected for interstellar extinction and underlying stellar absorption $I(\lambda)$are presented in Table 2. All lines have been normalized to the H$\beta$ intensity. The H$\beta$ equivalent width EW(H$\beta$), the absorption equivalent widths EW(abs) of the Balmer lines, the H$\beta$ flux, and the extinction coefficient C(H$\beta$) (this is a sum of internal extinction in HS 0822+3542 and foreground one in the Milky Way) are also shown there.

For the simultaneous derivation of C(H$\beta$) and EW(abs), and to correct for extinction, we used a procedure described in detail in Izotov, Thuan & Lipovetsky ([1994]). The abundances of the ionized species and the total abundances of O, Ne, N, S, and He have been obtained following Izotov, Thuan & Lipovetsky ([1994], [1997b]) and Izotov & Thuan ([1999]).