2 Selection

Most of the systems presented here were selected by the criterion that results from Tycho-2 disagreed to some extent with Hipparcos, implying that we now had some useful new information available. We selected stars where Tycho-2 positions deviated more than $0\hbox{$.\!\!^{\prime\prime}$ }8$ or $0\hbox{$.\!\!^{\prime\prime}$ }5$from the Hipparcos values for double and single stars respectively. Also cases where a Tycho star lies within 20 arcsec of a Hipparcos star are interesting especially if no astrometric solution was obtained for the Hipparcos Catalogue. The limit of 20 arcsec corresponds roughly to the radius of the Instantaneous Field of View in the Hipparcos main instrument ([ESA 1997]). We also added those Hipparcos double stars which have a note in the DMSA/C annex, provided Tycho-2 contained at least two stars within a reasonable distance. This group consisted mainly of cases where Hipparcos offers a choice of two solutions for a system. From this selection we then subtracted the systems studied by [Söderhjelm (1999)] because we did not expect to be able to add much to his analysis and by necessity we subtracted stars where Transit Data are not published. We also avoided cases where an improved solution is given in the General Notes in the Hipparcos Catalogue. The resulting list of 813 systems had only 22 systems in common with [Falin & Mignard (1999)] which we decided to keep. Finally we added a few close binaries, recently resolved by Mason et al. ([1999]).