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Astron. Astrophys. 338, 1031-1040 (1998)

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1. Introduction

The suspected light variability of HD 81410 (=IL Hya) by Cousins & Stoy (1963) was confirmed by Eggen (1973) who observed the star in 1971 and 1972, and found it to be a variable with an amplitude of about 0.5 mag in V. The light curves obtained during the two observing seasons differed appreciably, and a period of 25.4 days was found to satisfy the observations of each seasons separately. Bidelman & MacConnell (1973) listed HD 81410 as a star with K1III spectrum displaying strong [FORMULA] & K emissions and filled Balmer lines . The radial velocity measurements made independently by Wayman and Jones (Eggen 1973) showed HD 81410 to be a single-lined spectroscopic binary.

Raveendran et al. (1982), who observed HD 81410 almost a decade after Eggen (1973), found that the photometric period is in fact close to 12.87 days. They found that the amplitudes of light variation in both B and V had changed drastically during the intervening period, and were only around 0.15 mag. From the observed ([FORMULA]) and ([FORMULA]) colours of HD 81410 at its light maximum, they suggested that the secondary component of HD 81410 is probably an F dwarf. Fekel et al. (1986) found HD 81410 to show very strong [FORMULA] & K emission features typical of a late type chromospheric active star, and no evidence for a uv continuum from an F type companion, as suggested by Raveendran et al. (1982). Cutispoto (1993) assigned the spectral types K2-3IV and G4V to the components from the observed colours of HD 81410. The secondary component was detected and its radial velocity measured by Donati et al. (1997) on two high resolution, high S/N spectra taken in December 1993. Recently, Weber & Strassmeier (1998) have reported 12 radial velocities for the secondary component and derived its amplitude; they suggest a spectral type of mid to late F main sequence or G0V-IV for it.

The Li I 6708 Å absorption line is very strong in the spectrum of HD 81410. The equivalent width of the line, which does not vary significantly with the photospheric spot visibility, indicates a rather high value of log n(Li) = 1.34, and such a high value is most likely due to a real decrease in Li depletion in HD 81410 (Pallavicini et al. 1992, 1993)

Slee et al. (1984) detected HD 81410 as a highly variable radio source; during the initial phases of the major flare observed on 2 August 1983, the flux density at 5 GHz increased from about 15 to 28 mJy in 1.5 hours, almost by a factor of 2. HD 81410 has strong EUV emission; it is one of the 383 objects contained in the Bright Sources Catalogue compiled from the results of the recent ROSAT-WFC all sky survey in the 60-200 eV energy band (Pounds et al. 1993).

In this paper we present new photometry of HD 81410, and discuss its long-term photometric behaviour. We also discuss the implications of the results of photometric observations of active RS CVn stars on their surface distribution of starspots. Further, we model a set of light and colour curves of HD 81410 obtained at closely spaced epochs to investigate the short-term evolution of spots.

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© European Southern Observatory (ESO) 1998

Online publication: September 17, 1998