photon count records. There is no simple way to correct for this effect in TGAS, nor was it included in our simulations. However, we argue that its impact on the TGAS results should be very small. The [*Tycho*]{} positions are mainly used to derive the proper motions on a baseline of 24 years. Since the parallax of a given star is typically of similar size as its annual proper motion, and the position offset is just a fraction of the parallax, it follows that the resulting annual proper motion is typically only offset by a few per cent of the parallax. This, in turn, should have an almost negligible impact on the parallax, which is mainly derived from the [*Gaia*]{} observations relative to the extrapolated linear motion.

Reference frame of TGAS
-----------------------

The [*Tycho*]{} positions around 1991 and the [*Gaia*]{} observations around 2015 are by themselves not sufficient to determine the spin of the reference frame for TGAS, only its orientation at the [*Tycho*]{} epoch. By incorporating positions and proper motions from [Hipparcos]{} in TGAS, in the same way as described in the HTPM paper, the TGAS results are effectively on the [Hipparcos]{} reference frame.

Conclusions
===========

The currently foreseen 