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Astron. Astrophys. 350, 1085-1088 (1999)

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1. Introduction

After the initial identification of the optical counterparts of X-ray sources, it became apparent that in many cases the visible star was a binary companion of the object emitting the X-rays. The distance to these systems can be derived from the visible star's spectroscopic parallax if one makes the usual assumption that the star's luminosity is normal for its spectral type. It is possible in theory for a star with an unusual structure to mimic the type of spectrum seen in the optical counterparts, leading to an incorrect spectroscopic parallax for these systems (Wilson & Fox 1981; Bahcall et al. 1974; Trimble et al. 1973). Hence, we proposed including several optical counterparts of X-ray sources in the Hipparcos mission in order to determine whether their astrometric parallaxes were consistent with their spectroscopic parallaxes. The Hipparcos catalog (ESA 1997) includes eight of the systems we proposed. Given that six of these systems had spectroscopic parallaxes of [FORMULA] kpc and the nominal precision of Hipparcos parallax measurements was [FORMULA] mas, we expected the measured parallaxes of these six systems to be consistent with zero.

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© European Southern Observatory (ESO) 1999

Online publication: October 14, 1999