1. Bacchus, P. and Lacroûte, P.: 1974, in ?New problems in Astrometry?, I.A.U. symp. 61, W. Gliese, C. A. Murray and R. H. Tucker, ed., p. 277.
2. Barbieri, C.: 1979, Ed. ?European astrometry satellite?, colloquium in Padova, 5?7 June 1978, in press.
3. E.S.A.: 1978, ?Space astrometry, HIPPARCOS?, report on phase A study, E.S.A. publication DP/PS (78) 13.
4. Høg, E.: 1975, in ?Space astrometry?, proceedings of the Frascati symposium, R. Pacault ed., p. 59.
5. I.A.U.: 1976,Transac. Int. Astron. Union,XVI A, part 1, G. Contopoulos, ed., p. 224.