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Astron. Astrophys. 364, 706-711 (2000)

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2. Observations

2.1. Photometry

Our photometric study on HD 172468 and HK Dra is based on the observations collected by the 80-cm Automated Photometric Telescope (APT-80) at the " M. G. Fracastoro" station of Catania Astrophysical Observatory on Mt. Etna (Italy), and by the Hipparcos satellite.

The APT-80 feeds a single channel charge-integration photometer equipped with an uncooled Hamamatzu R1414 SbCs photomultiplier and Johnson's standard UBV filters. From the fall of 1992 the APT-80 is being devoted to the systematic photometric monitoring of a selected sample of known or suspected chromospherically active stars (see e.g., Messina 1998).

HD 172468 (check star 1) and HK Dra (check star 2) were observed by the APT-80 from 1993 through 1999 differentially with respect to the primary comparison star (c) HD 172268 (K5; V=7.89; B-V=1.25) and the check star (ck3) HD 169028 (K1III; V=6.29; B-V=1.10). These stars were all included in the observational sequence of the primary variable star target BY Dra. The observations were corrected for atmospheric extinction and converted into the Johnson's standard UBV system. The typical errors of the APT-80 differential photometry are about 0.008, 0.004 and 0.003 mag for the U, B and V filters, respectively. The differential values were finally averaged to obtain one single data point for each night in each filter. The standard deviation of the whole data set of ck3-c differential magnitudes clearly showed that these two stars were not variable.

The Hipparcos [FORMULA] photometry was obtained from 1990 through the beginning of 1993. [FORMULA] magnitudes were transformed into the Johnson V magnitudes by applying the correction: [FORMULA] mag for HD 172468 and [FORMULA] mag for HK Dra using the [FORMULA]V versus V-I relation taken from Perryman et al. (1997).

2.2. Spectroscopy

Our spectroscopic observations were contemporarily performed at the 91-cm Cassegrain telescope of Catania Astrophysical Observatory feeding a REOSC echelle spectrograph. We used an echelle cross-dispersion configuration yielding a resolution of [FORMULA]. The spectra were recorded on a back-illuminated [FORMULA] pixel CCD camera with [FORMULA] pixel size. In each frame five orders were recorded, covering the spectral range from about 5860 to 6700 Å and including the H[FORMULA] and Na I D1, D2 absorption lines.

A spectrum of HD 172468 was obtained on April 28, 1999. The HK Dra spectra were obtained on April 7 and 28, and June 3, 1999. The signal-to-noise ratio (S/N) was between 50 and 90 at H[FORMULA], depending on the exposure time (900-1800 s). A standard data reduction was performed by using the ECHELLE task of IRAF package.

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001