Photocentric orbits from a direct combination of ground-based astrometry with Hipparcos II. Preliminary orbits for six astrometric binaries

Based on a direct combination of the Hipparcos data with astrometric ground-based observational catalogues having epochs between 1938 and 1999 the preliminary orbits and component masses are calculated for six binaries with no previous orbit calculation: ιι Vir (HIP 69701) with period of 55 years, photocentric semi-major axis of 200 mas, relative semi-major axis of 830 mas and a dwarf secondary of 0.6 solar masses; γγ UMa (HIP 58001) – 20.5 years, 90 mas, 460 mas and a dwarf secondary of 0.8 solar masses; κκ Del (HIP 101916) – 45 years, 100 mas, 520 mas and a dwarf secondary of 0.4 solar masses; 20 Oph (HIP 82369) – 35.5 years, 140 mas, 460 mas and a dwarf secondary of 0.8 solar masses; μμ Ser (HIP 77516) – 36 years, 110 mas, 350 mas and a secondary of 2.3 solar masses; as well as a possible new component of Mizar A (ζζ UMa, HIP 65378) – 36.5 years, 180 mas, 780 mas, 1.5 solar masses. The latter may be a pair of dwarfs.