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Astron. Astrophys. 342, 167-172 (1999)

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On the period history of [FORMULA] Cygni

C. Sterken *  1, E. Broens 2 and C. Koen 3

1 University of Brussels (VUB), Pleinlaan 2, B-1050 Brussels, Belgium
2 Vereniging Voor Sterrenkunde, Werkgroep Veranderlijke Sterren, Wateringstraat 143, B-2400 Mol, Belgium
3 South African Astronomical Observatory, P.O. Box 9, Observatory, 7935 Cape, South Africa

Received 1 October 1998 / Accepted 3 November 1998


We discuss the light variability of the Mira variable [FORMULA] Cygni, present a homogeneous set of times of minimum and maximum light since its discovery date in 1686, and analyse the variations of the pulsation period. There is a substantial cycle-to-cycle variation in the pulsations (standard error [FORMULA]) with quite strong evidence for a linearly increasing period. Quasi-cyclic patterns in the [FORMULA] diagrams are visible, but there is no compelling evidence that these apparent cyclic period changes are real since the residuals left after fitting a linear period increase plus intrinsic period scatter have properties consistent with being white noise. The Lombard frequency domain test for a changing period applied to the residuals also gives unconvincing significance levels. The low gradient of the luminosity change with period is compatible with the star undergoing a rise in luminosity when H shell-burning takes over.

Key words: stars: individual: HD 187796 – stars: oscillations – stars: AGB and post-AGB

* Belgian Fund for Scientific Research (FWO)

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© European Southern Observatory (ESO) 1999

Online publication: December 22, 1998