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Astron. Astrophys. 351, 212-224 (1999)

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1. Introduction

The most distinctive aspects of sunspots, like the sunspot cycle, the latitudinal migration of activity (i.e. the "butterfly" diagram), the solar differential rotation, and the unpredictable prolonged activity suppressions, were already discovered by Schwabe (1843), Carrington (1858) and Maunder (1890, 1894). Yet, the correlation between solar magnetic activity and luminosity could be verified only recently (e.g. Willson & Hudson 1991). That starspots, being the counterparts of sunspots, induce detectable rotational and long-term luminosity variations in some late-type stars was confirmed earlier (e.g. Kron 1947; Hall 1972). The FK Comae stars are one of the 11 groups of stars, where magnetic activity has been observed (Hall 1991). Bopp & Rucinski (1981) defined this group, possibly representing coalesced W Uma binaries, as single and rapidly rotating G-K giants with strong chromospheric and transition region UV emission. The rapidly rotating giant V 1794 Cyg fulfills all FK Comae-type classification criteria: G5iii-iv, [FORMULA] km s-1 (i.e. single), [FORMULA], and [FORMULA] km s-1 (Herbig 1958; Huenemoerder 1986; Jetsu et al. 1990a; Fekel 1997). The detections of chromospheric, transition region, and coronal activity include emission at radio (Drake et al. 1990; Walter et al. 1990), IR (Dempsey et al. 1993), Caii H, H[FORMULA] (Huenemoerder 1986), UV (Bopp & Stencel 1981; Simon & Fekel 1987), and X-ray (Schachter et al. 1996) wavelengths. The common denominator between V 1794 Cyg and the Sun is magnetic activity. But in terms of the solar-stellar-connection, the rapidly rotating giant V 1794 Cyg is several factors more active than the Sun, as predicted by numerous rotation-activity-relations (e.g. Simon & Fekel 1987; Hartmann & Noyes 1987; Pasquini et al. 1990; Strassmeier et al. 1990; Böhm-Vitense 1992). We perform a detailed time series analysis of the starspot induced luminosity variations in V 1794 Cyg, and magnetic activity connected phenomena, such as flares, activity cycle(s), differential rotation and active longitudes.

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© European Southern Observatory (ESO) 1999

Online publication: November 2, 1999