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eJAAVSO

Microvariability of Red Giant Stars

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N. Mowlavi
Observatoire de Genève, CH-1290 Versoix, Switzerland

A. Jorissen
Chercheur Qualifiè F.N.R.S., Institut d'Astronomie et d'Astrophysique, U.L.B., Boulevard du Triomphe, CP 226, B-1050 Bruxelles, Belgium

Abstract
Light variations are a common feature of red giant stars. Recent studies have shown that the amplitude of the light variations steadily increases along the giant branch, from microvariable and irregular K giants to semiregular and Mira- type M giants. Recent accurate photoelectric studies actually suggest that all stars among late K and M giants are variable to some degree. In particular, Jorissen et al. (1997) provide evidence that there is a minimum variability amplitude for a given spectral type, which increases with decreasing stellar surface temperature (or later spectral type). We summarize their results in this paper. The first light curves of microvariable K giants, with semi-amplitudes ~10 millimag (in the Stroemgren y band) on time scales of 5 to 10 days, are also presented. Such patterns could characterize all microvariable K giants, but need tight time sampling and high photometric accuracy to be revealed.

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