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Astron. Astrophys. 327, 231-239 (1997)

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2. Data

2.1. Time-resolved spectroscopy

WW Cet was observed with the Image Tube Spectrograph (ITS) combined with the Reticon Photon Counting System (RPCS) mounted on the 1.9 m telescope of the South African Astronomical Observatory in Sutherland, South Africa. In total, 67 measurements have been obtained by one of us (WFW) in 1993 (October 10 and 11) and in 1994 (October 12 and 13). The RPCS collects data in two channels, one containing the object, the other the sky background. These channels have been switched frequently. For our measurements, grating #5 has been used with a dispersion of 50 Å/mm and a total range of 800 Å, centered on H [FORMULA].

Observations were done with a 300 [FORMULA] slit ([FORMULA] 1".8), giving a resolution of 1.5 Å. After every 4 exposures, a Ne spectrum was taken for wavelength calibration. Object exposure times were 600-720 s in 1993, and 420 s in 1994. These values were chosen to minimize phase smearing (600 s correspond to 0.04 orbital periods). The S/N for a single spectrum is rather poor, about 7 in 1993 and about 5 in 1994.

The reductions have been done with IRAF1 , using the noao-onedspec package. Both channels have been divided by their corresponding flat fields, which were normalized by fitting Chebyshev functions of high order. The background data was then subtracted from the object data. The wavelength calibration of the spectra resulted in a mean rms of 0.014 Å, or 0.03 pixel.

Additionally, standard stars have been observed every night, and we therefore also attempted a flux calibration. However, testing our fluxes yielded possible errors as large as 40% due to non-photometric conditions during the observations. We therefore fitted the continuum with low-order Chebyshevs and used these normalized spectra for the further analysis in Sect. 3.1.

2.2. Photometry

We have used the Dutch 0.9 m telescope at the European Southern Observatory, La Silla (Chile) on October 8-10, 1995. We have taken CCD frames in B and V filters with exposure times of 60 and 30 seconds, resp. In the three nights we obtained 10.17 hours of data, but only during the first night we could cover a whole orbital period.

The data of WW Cet have been reduced with respect to a reference star nearby the object, yielding differential photometry with [FORMULA] 0.01 magnitudes. Comparison with the data on secondary photometric standards published by Misselt (1996) showed that WW Cet was in quiescence during the observations, with V [FORMULA] 14.2. Cragg et al. (1995) report the system in outburst on September 28, 1995, but remaining in quiescence during October. Mean magnitudes for the three nights are given in Table 1. All photometric data points are shown in Fig. 1.


Table 1. Mean photometric values for WW Cet. The errors give the mean sigma during the night.

[FIGURE] Fig. 1. B photometry for the three nights in 1995: Oct 8 ([FORMULA]), 9 ( [FORMULA]), and 10 ([FORMULA]). Magnitudes have been calculated by using calibrated data on comparison stars provided by Misselt (1996). The lines in the lower half of the plots give the B-V behaviour, which was calculated by interpolating the respective photometries. These data have been smoothed and an offset of 15.2 magnitudes has been applied

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© European Southern Observatory (ESO) 1997

Online publication: April 8, 1998