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Issue A&A
Volume 398, Number 2, February I 2003
Page(s) 541 - 550
Section Stellar clusters and associations
DOI http://dx.doi.org/10.1051/0004-6361:20021624



A&A 398, 541-550 (2003)
DOI: 10.1051/0004-6361:20021624

Variable stars in the globular cluster M 13

G. Kopacki, Z. Kolaczkowski and A. Pigulski

Wroclaw University Observatory, Kopernika 11, 51-622 Wroclaw, Poland
(Received 31 July 2002 / Accepted 30 October 2002)

Abstract
Results of a search for variable stars in the central region of the globular cluster M 13 are presented. Prior to this study, 36 variable and suspected variable stars were known in this cluster (Osborn 2000; Clement et al. 2001). Of these stars, five were not observed by us. We find v3, v4, v10, v12, and v13 to be constant in light. Surprisingly, only two out of the ten variable star candidates of Kadla et al. (1980) appear to be variable. Both are RRc variables. Additionally, three RR Lyrae stars and one SX Phoenicis variable are discovered. Three close frequencies are detected for an RRc star v36. It appears that this variable is another multi-periodic RR Lyrae star pulsating in non-radial modes. Light curves of the three known BL Herculis stars and all known RR Lyrae stars are presented. The total number of known RR Lyrae stars in M 13 is now nine. Only one is an RRab star. The mean period of RRc variables amounts to $0.36\pm0.05$ d, suggesting that M 13 should be included in the group of Oosterhoff type II globular clusters. Mean V magnitudes and ranges of variation are derived for seven RR Lyrae and three BL Herculis variables. Almost all observed bright giants show some degree of variability. In particular, we confirm the variability of two red giants announced to be variable by Osborn (2000) and in addition find five new cases.


Key words: stars: population II -- stars: variables: RR Lyr -- stars: variables: Cepheids -- globular clusters: individual: M 13

Offprint request: G. Kopacki, kopacki@astro.uni.wroc.pl

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