2 Observations and data reduction

2.1 Spectroscopy

The spectroscopic observations of FS Aur were obtained on January 18, 1996 with the 6 m telescope at the Special Astrophysical Observatory. The SP-124 spectrograph was used with a PHOTOMETRICS CCD, which has 1024$\times$1024 pixels. The seeing was around 2 arcsec and we selected a slit width of 1 $\hbox{$.\!\!^{\prime\prime}$ }$5. The spectra have a resolution of 2.6 Å, covering the range 4100-5000 Å. About 1.5 orbital cycles were covered with 29 spectra of 200 s long exposure (dead time between exposures was 30 s). He-Ne-Ar lamp exposures were taken typically every 30 min.

The spectra were reduced in the standard manner. All CCD frames were debiased, flat-fielded and wavelength calibrated using the MIDAS system. For the wavelength calibration of the spectra, interpolation was used between neighbouring arc spectra. The root mean square of the polynomial fits is $\sim$0.030 Å. One-dimensional spectra were extracted using the optimized algorithm proposed by Horne (1986). The resulting spectra were reduced to an absolute flux scale by calibration with standard star G191b2, which was observed in the same night. Note that substantial light losses at the slit edges occur. Therefore, our data cannot be used to obtain absolute fluxes though the obtained fluxes are good to approximately 10%. In view of this, from now on we shall use spectra normalized to the continuum.

2.2 Photometry

The photometric observations were obtained with the 1 m telescope at the Special Astrophysical Observatory (Nizhnij Arkhyz, Russia), using a 1024$\times$1024 CCD camera. 31 exposures in the V filter with 180 s integration time were obtained on February 12, 1997. The total observation time was more than 2 hours. Additional V-band CCD observations were carried out on October 11, 1997 during 4 hours using the same telescope with a 530$\times$580 CCD camera. A total of 34 frames were recorded with an integration time of 300 s.

All CCD images were debiased and flat-fielded using the MIDAS system. We calculated magnitudes of the source with respect to several "comparison'' stars within the field of view from Misselt (1996), using aperture photometry (we used standard stars C₁, C₃ and C₄). The differences between daily averages of the differential magnitudes (C₁-C₃, C₄-C₁, C₄-C₃) were within 0.01 mag during our observational runs.

Figure 1: The single typical normalized spectrum (top panel) and the average spectrum (bottom panel) of FS Aur. The mean spectrum is an average of all spectra, corrected for wavelength shifts due to orbital motion.

 

 

Table 1: Equivalent width (EW), Full Width at Half Maximum (FWHM), Full Width at Zero Intensity (FWZI) and Relative Intensity of the major emission lines.

Spectral line	EW	FWHM	FWZI	Relative
	(Å)	(km s-1)	(km s-1)	Intensity
H$\beta$	23.2	950	3900	2.16
H$\gamma$	21.7	1000	3500	2.03
H${\delta}$	14:	1000	3500:	1.81
HeI $\lambda4471$	6.7	1100	2700	1.36
HeI $\lambda4713$		1000		1.10
HeI $\lambda4921$	3.4	1050	2500	1.18
HeII $\lambda4686$	6.4	1300	3500:	1.29
CIII/NIII $\lambda4650$	1.9	1270:		1.10