Astrometric search for unseen stellar and sub-stellar companions to nearby stars and the possibility of their detection

Unseen companions to nearby stars are found astrometrically through perturbations in the proper motion from photographs taken with long-focus telescopes. The number of known unseen astrometric companions to nearby stars with photocentric orbits has grown by thirty percent in the last few years. Individual cases are discussed and optimum epochs given for resolution of the components. Orbital analysis of the photocentric positions on the photographic plates provides all information for accurate mass determination of the components except for m and angular separation, , of the two components which must come from another technique. There are potentially thirty low luminosity stars including some likely sub-stellar objects whose masses could be instantly found with the observations of these additional two parameters.A list of the stars known within five parsecs as of 1978 July is given and the status of unseen companions to these stars is discussed on the basis of long interval astrometric coverage.