Spectral-line variations in Sigma Geminorum

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Abstract
Sigma Gem is a bright RS CVn binary with an orbital period of 19 day. Photometric observations by Fried et al. show that spots appear in two zones of longitude on opposite sides of the K1 giant primary. High-resolution (0.16 Å) echelle-Reticon spectra of σ Gem in the Hα order with high signal-to-noise ratio (∼130) show that the core of Hα varies with orbital phase in agreement with the spot zones and that the Hα brightness of a zone changes in time. Neutral metal lines in the same order have been studied to investigate the properties of the spots. A new method is described to determine spot areas and temperatures from temperature sensitive metal lines. The lack of observed line variation in the normal spot zones requires a temperature difference between spot and normal photosphere greater than 1000 K and spot area less than 20 per cent of the disc. In an unusual event, a short-lived spot appeared in the region between zones. For this spot the temperature difference was about 500 K.

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