RELATIVE ASTROMETRY WITH HIPPARCOS - DOUBLE STARS WITH ORBITAL MOTION

During the Hipparcos data analysis an algorithm was developed to study orbital motion in double stars and to derive the seven Dommanget para meters in the positive cases. Three approaches were followed: linear m otion model, quadratic motion model and orbital model. Ln the last mod el the algorithm needs sufficiently good starling values and it has be en applied to a set (756) of known orbital doubles. 600 of the obtaine d solutions have been considered acceptable for further detailed analy sis and critical discussion. It turned out that for long period orbits the results are useful just as an estimate of relative positions at t he Hipparcos mission lime, while for short periods the whole reconstru cted orbit can assume interesting physical meaning. The method is desc ribed and the results are globally discussed.