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Astron. Astrophys. 363, 1019-1025 (2000)

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1. Introduction

Among visual binaries with variable components, late type young pre-main sequence (PMS) stars (T Tau, flare stars) constitute an interesting subclass of newly formed stellar systems. It is known that multiplicity is rather common among these stars (Evans 1977). Recent studies of a number of T Tau stars in various star-forming regions confirmed the presence of companion stars bound gravitationally to the primary star (Ghez et al. 1995; Simon et al. 1993). The binary star frequencies suggest that T Tau stars are 2 to 4 times more likely to have a companion in the separation range from 1-150 AU (Simon et al. 1992; Simon 1995) than solar-type main sequence (MS) stars. As regards wider separations (150 -1800 AU), this trend is not so evident (Brandner et al. 1996) despite the excess of PMS binaries found by Reipurth & Zinnecker (1993).

The prevalence of binary and multiple systems at early stages of stellar evolution shows that a system of this kind is the most probable outcome of the star-forming process. Under the assumption that PMS and MS binary frequencies should be the same, such excess implies that binary orbits in the star forming regions undergo secular evolution towards the MS. The increasing number of such systems, as well as precise knowledge of their orbital and astrophysical parameters, enable investigators to constrain the star formation models as well as indicate their possible evolutionary tracks.

It is worth noting that the study of relationships between dynamical and astrophysical parameters in binary systems represents one of the most interesting research fields in modern astrophysics, especially in view of the growing spatial resolution of the largest telescopes with an unprecedent capacity (milliarcseconds) to resolve extremely close systems. This allows unbiased measurement of physical parameters for each component separately.

One must note, however, that often astrophysical parameters are determined or derived for many stars as a single object while in reality they are double (or sometimes even multiple) systems. In such cases, derived parameters may serve only as orientative values, without describing real physical properties of any concrete component.

In the course of our ongoing study of late type visual binaries with variable components (Tamazian et al. 1997, 1999) we have observed a number of systems with K and M type primaries with either known or suspected, but not yet confirmed, (or characterized) variability.

This article presents the results of BV photometric monitoring, spectrum, speckle spectroscopy data, speckle interferometric and polarimetric observations of the visual binary WDS 00550+2338 ([FORMULA]HD 5286[FORMULA]ADS 755), which is also catalogued as NSV 343 in the New Catalogue of Suspected Variable Stars (Kukarkin et al. 1982).

The spectral type K1 IV, corresponding to the composite spectrum, is known (Abt 1981; Keenan & McNeil 1989). According to Edwards (1976), its components A and B are late type G6 IV (V=6.1) and K6 IV (V=6.7) stars respectively, separated by [FORMULA].88 (epoch 2000.0; orbit of Docobo & Costa 1990). It is worth noting that Edwards (1976) assigned aforementioned spectral types not on the basis of direct observations but by using an empirical method he elaborated to separate the components in close binary systems.

With Hipparcos (Perryman 1997) parallax [FORMULA].02569 ([FORMULA] = [FORMULA].00129) we obtain a distance 39 pc for this star.

The third component C catalogued in ADS (Aitken 1932) is much more fainter (V[FORMULA]11.0) and distant ([FORMULA]). It is unlikely to be related physically with the AB pair.

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© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000