Astron. Astrophys. 327, 173-182 (1997)

2. Observation and data reduction

2.1. Spectroscopy

173 spectra were recorded at the Calar Alto Observatory in Spain with the TWIN spectrograph at the 3.5m telescope. During each exposure, a blue and a red spectrum were obtained simultaneously, covering the spectral range between 3440 and 8910Å. For the first observing run in 1992, the GEC#14-CCD (1155 768 pixel) was used in the blue and the RCA#10-CCD (1024 640 pixel) in the red range. During this observing run, the system was in quiescence. For the second campaign in 1993, the CCDs were replaced by the TEK#11-CCD and the TEK#12-CCD (both 1024 1024 pixel), respectively. This time, the system was observed during an outburst. The journal of observations is given in Table 1.

Table 1. Journal of observations of all spectra. The integration time varies between 200s...1200s.

The optimal extraction algorithm given by Horne (1986) was applied to reduce the spectra. This procedure includes the removal of bad pixels and of cosmic ray hits and also takes into account the characteristic bias, the dark current, the pixel-to-pixel-variation, the blaze-function, and the scattered light. For wavelength calibration a helium-argon source was used. This calibration was checked using sky-lines which were superimposed on all exposures. Typical spectra obtained in quiescence and in outburst are displayed in Fig. 2 and Fig. 3. During an outburst, additional emission lines of C II, C III, N III, He I, and He II appear between 4700 and 4900Å, as well as the C II emission line at 4262Å and the N II emission line at 6340.67Å. During quiescence, seven Ca I absorption lines in the wavelength range between 6100 and 6500Å are visible which obviously follow the motion of the secondary (Fig. 4). All lines identified in the spectra of HS 1804 + 6753 are compiled in Table 2 to 4 and 7.

Fig. 2. Blue spectral range of HS 1804 + 6753. Before averaging, the individual spectra were shifted according to the radial velocity to emphasize the lines moving with the primary component. The mean spectrum during quiescence is displayed in the upper panel, that during outburst in the lower panel. Dips in the spectra which are caused by emission lines of the flatfield light source are marked with FF. Intensities are given in arbitrary units.

Fig. 3. Red spectral range of HS 1804 + 6753. Before averaging, the individual spectra were shifted according to the radial velocity to emphasize the lines moving with the primary component. The mean spectrum during quiescence is displayed in the upper panel, that during outburst in the lower panel. Atmospherical absorptions are marked with "at." . Intensities are given in arbitrary units.

Fig. 4. Radial velocity variations of the Ca I absorption lines of the secondary component. 20 spectra are arranged in sequence of increasing orbital phase. The phase interval is repeated for clarity.

Table 2. Emission lines of the primary. Lines which are possibly blended are marked with a 'B'. For the balmer lines and two He lines equivalent widths are given in brackets.

Table 3. Absorption lines of the primary.

Table 4. Spectral lines of the primary, showing absorption features as well as emission features.

Table 5. Journal of the photometric observations (1991-1993). Abbreviations: Observations at Wendelstein: W; Observations at Calar Alto: C; Observations during an outburst: O.

Table 6. Journal of the photometric observations (1996). Abbreviations: Observations at Wendelstein: W; Observations at Calar Alto: C; Observations during an outburst: O.

Table 7. Absorption lines of the secondary.

2.2. Photometry

High-speed simultaneous UBVRI photometry was performed between April 1991 and February 1996 with the 2.2m telescope at Calar Alto and with the 0.8m telescope at Wendelstein Observatory. The total observing time amounts to 330 hours. In all cases the Multichannel-Multicolour Photometer (MCCP) (Barwig et al. 1987) was used. This high-speed photometer consists of three fiber channels so that the program star, a nearby comparison star, and the sky background can be measured simultaneously. Each of the three channels feeds a prism spectrograph in order to split the light into five colours, which nearly fit the Kron Cousins UBVRI photometric standard system. During all observations, the integration time was set to 2 s. The raw data was reduced by applying the standard reduction procedure. For details see Barwig et al. (1987). The journal of observations is listed in Table 5 and 6. Typical UBVRI light curves of HS 1804 + 6753 in quiescence and in outburst are displayed in Fig. 5 and Fig. 6.

Fig. 5. Typical UBVRI light curves of HS 1804 + 6753 during quiescence measured relative to a comparison star. The photometric data were recorded with the MCCP at Calar Alto Observatory on 4.7.1993. Each data point represents an integration of 2s.

Fig. 6. Typical UBVRI light curves of HS 1804 + 6753 during outburst measured relative to a comparison star. The photometric data were recorded with the MCCP at Wendelstein Observatory on 30.12.1991. Each data point represents an integration of 2s.

© European Southern Observatory (ESO) 1997

Online publication: April 8, 1998
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