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Astron. Astrophys. 355, 227-235 (2000)

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1. Introduction

Chromospheric and coronal heating on the Sun is highly concentrated into localised active regions of enhanced magnetic field. The distribution of such active regions with solar longitude is highly non-uniform, with the global brightness in high-temperature spectral features sometimes being dominated by a few very active regions. This is especially true near the maximum of the solar magnetic cycle.

Chromospherically active late-type stars exhibit most of the characteristics of the active Sun but on a globally much enhanced scale. RS CVn stars are close late-type binaries in which one component lies above the main sequence and, forced into corotation, is chromospherically active as a result of dynamo generated magnetic fields. RS CVn's exhibit a wide range of solar-like activity phenomena. These include non-radiatively heated chromospheres and X-ray emitting coronae (Doyle et al. 1992a; Doyle et al. 1992b), cool surface spots (Byrne 1992a; Byrne 1992b) and frequent flares (Doyle et al. 1989b).

Based on the solar experience, it might be expected that non-uniform distributions of magnetic heating in RS CVn stars would lead to variability in the stellar fluxes in suitable chromospheric and coronal lines as the star rotates, i.e. rotational modulation. Such effects have been very elusive, however, in spite of much observational effort (Rodono et al. 1987; Byrne et al. 1987; Byrne et al. 1989; Doyle et al. 1989b; Doyle et al. 1989a). However, since most previous efforts have been based on either sampling a single rotation of the active star, or random sampling during many different rotations, there is an obvious risk of any rotational modulation being masked by short-term variability, such as flaring, or longer-term variations, such as the growth and decay of active regions.

In this paper we describe observations of the 6.72d period SB1 RS CVn K2IV binary, II Peg with the IUE satellite. High and low resolution spectra were taken during two stellar rotations in order to examine the above mentioned issues. Note that, as in Paper I, we use the orbital ephemerides of Vogt (1981), i.e. JD =2443033.47+6.72422E, which we found according to Byrne et al. (1995) to be more accurate than any of the other published ephemerides. We also assume that the axial rotation of II Peg is tidally locked to the orbital motion of its companion.

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© European Southern Observatory (ESO) 2000

Online publication: March 17, 2000