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

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1. Introduction

HD 172468 ([FORMULA]7.49; B-V[FORMULA]1.28, V-[FORMULA]) is classified in the SIMBAD database as a K2 star and is listed in the Hipparcos Catalogue (HIP 91329) with a parallax of 3.11 mas (Perryman et al. 1997). HD 172468 along with HD 172268 have been generally used as comparison stars for differential photometric observations of the variable BY Dra, which is the prototype of the class of spotted and chromospherically active late-type dwarf stars (Bopp & Fekel 1977). However, Vogt (1981), Oláh & Soliman (1984) and Panov & Ivanova (1993) noted that their V-band differential magnitudes were variable within 0.03 mag. Such small variability could be attributed to one or both stars; however, Oláh & Soliman (1984) suggested that HD 172468 was most likely the star responsible for the observed variability because of its late spectral type (K2) and sound suspects on the presence of some kind of activity.

HK Dra ([FORMULA]HD 171911; [FORMULA]; B-V[FORMULA]1.60, V-[FORMULA]) is a giant star classified in the Perkins Catalogue of revised MK types as M4III-IIIb (Keenan & McNeil 1989). Its radial velocity of [FORMULA] km s-1 is reported as constant in the Wilson-Evans-Batten Catalogue (WEB) (Duflot et al. 1995). HK Dra is listed in the Hipparcos Catalogue (HIP 91061) as a variable star of irregular type with a parallax of 3.94 mas (Perryman et al. 1997). However, systematic photometric observations had never been carried out. In a recent study of M giants observed by Hipparcos (Dumm & Schild 1998) a radius of [FORMULA] and a mass of [FORMULA] were inferred. HK Dra was included in an extensive soft X-ray survey of 380 giants carried out with the Einstein satellite (Maggio et al. 1990), but it was not detected above the lower X-ray luminosity limit of [FORMULA] erg sec-1.

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001