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

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Serendipitous discovery of an irregular and a semi-regular type variable in the field of BY Draconis

S. Messina 1, G. Marino 2, M. Rodonò 1,3 and G. Cutispoto 1

1 Catania Astrophysical Observatory, Via S. Sofia 78, 95125 Catania, Italy
2 Telescopio Nazionale Galileo, Calle Alvarez de Abreu 70, 38700 Santa Cruz de La Palma, Canary Islands, Spain
3 University of Catania, Department of Physics and Astronomy, Via S. Sofia 78, 95125 Catania, Italy

Received 29 May 2000 / Accepted 9 October 2000


We present new evidence of the optical variability of two red giant stars: HD 172468 and HK Dra, based on photometric and spectroscopic observations. These stars had been included as check stars in our photometric monitoring program of BY Dra and turned out to be variable.

HD 172468, whereas almost constant for most of the time, suddenly started to drop in brightness to such a low level to become undetectable. We suspect that such an abrupt event may be an "obscurational" minimum, that is typical of eruptive RCB stars, or may be due to the variable extinction by circumstellar dust in a young Orion type object.

HK Dra, already known as an irregular variable, is characterised by periodic flux modulation with season-to-season changes of the photometric period, as inferred from a periodogram analysis. It also shows changes of the light curve peak-to-peak amplitude and shape. Such a behaviour in giant stars is commonly found among semi-regular giants (SR) at the Asymptotic Giant Branch (AGB). Our radial velocity measurements rule out that HK Dra may be a close binary system.

Key words: stars: AGB and post-AGB – stars: individual: HK Dra, HD172468 – stars: variables: general

Send offprint requests to: S. Messina (sme@sunct.ct.astro.it)

© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001