Astron. Astrophys. 336, L25-L28 (1998)

1. Introduction

Since the late sixties, when large cool spots were found on late-type active stars, and the seventies, when the reality of these spots was established, many questions still remain concerning the nature and evolution of these spots, as well as their lifetimes, activity cycles, latitude extent and correlation with other signatures of activity. Questions regarding active binaries with spotted components of the RS CVn type have been widely reviewed e.g. by Hall (1996 ). Analysis of long-term photometric observations of RS CVn stars using the few-spot approximation has established the following facts: (1) lifetimes of relatively small spots are proportional to their sizes, while lifetimes of relatively large spots are probably determined by the surface differential rotation; (2) the magnitude of the differential rotation is proportional to the period of the stellar rotation, and it decreases for shorter rotation periods; (3) there should exist long-term activity cycles similar to the solar 11-year sunspot or 22-year magnetic cycles, though cycle tracers are still under discussion. Active-longitude structures have been reported on some RS CVn stars (e.g. Henry et al. 1995 , Jetsu 1996 ). However, the discussion is not completed yet because of some contradictory results and a few non-consistent assumptions (see the discussion by Hall 1996 ).

Obviously, the long time span of photometric observations makes them attractive for searching long-lived active-longitude structures on spotted stars. Extensive photometric data have been used for spot modelling by Strassmeier (1990 ), Strassmeier et al. (1994 ) and Henry et al. (1995 ). In the present paper we used these observations for searching active longitudes on four RS CVn stars. However, we prefer to use the original light curves published in these papers. It is natural to suggest that the deepest minimum in the light curve is caused by the largest active region on the stellar surface consisting probably of a group of spots. The secondary minimum, if present, should correspond to another, smaller active region. Then, tracing the minima in time could provide information on migration of the active regions with no assumptions on their structures, shapes, areas, temperatures, and latitudes. Such an approach has been applied e.g. to data of FK Com by Jetsu et al. (1991 ) and revealed two active longitudes on its surface. One must note, that the technique of surface (Doppler) imaging, which has been applied to many RS CVn stars for the last decade, can be used for the same purpose, but the available time base of surface images for a particular star is short.

Recently, we obtained new surface images of one of the most active RS CVn star II Peg (Berdyugina et al. 1998b ). We found that two high-latitude active regions were dominant on the stellar surface in 1992-1996, which were interpreted as two long-lived migrating active longitudes. The period of the longitude rotation deduced from the images well coincides with the mean photometric period determined from the two-spot photometric curve modelling by Henry et al. (1995 ) for 1976-1992. Then, we found that spots in the earliest images well follow the latest photometric spots, and active regions can exist more than 6 years. Taking into account the results on II Peg, we decided to reanalyse all available long-term photometric observations for some RS CVn stars. Here, we report our first results for four stars of different periods of rotation: EI Eri (195), II Peg (671),  Gem (196), and HR 7275 (283).

© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998
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