as well as for the [Hipparcos]{} stars, thus avoiding the potential bias from auxiliary stars. Moreover, we find that such a solution is possible with even less [*Gaia*]{} data than required for HTPM. The resulting [*Tycho*]{}–[*Gaia*]{} Astrometric Solution (TGAS) could become the first full-sky astrometric solution using [*Gaia*]{} data, providing an important early validation of the instrument, calibration, and data processing, at the sub-mas level. Clearly the resulting parallaxes and proper motions of a few million [*Tycho*]{} stars are extremely interesting also from a scientific viewpoint, e.g. for local Galactic dynamics and cluster studies.

[*Tycho*]{} refers to the star catalogues derived from the star mapper instrument of the [Hipparcos]{} satellite. The original [*Tycho-1*]{} Catalogue [@1997ESASP1200.....P] gave positions and magnitudes for about 1 million stars. The later reduction, [*Tycho-2*]{} , extended this to about 2.5 million stars, almost complete to $V\la 11.5$, and with uncertainties of 5–70 mas at the mean epoch of observation ($\sim$1991.25). [*Tycho-2*]{} also gives proper motions, derived from a comparison with old photographic catalogues. These proper motions have uncertainties of a few mas yr$^{-1}$, but as they may contain systematic errors from the old data, they are not used in the TGAS solution.

![Fractions of the celestial 