SIMBAD references

Parallaxes and proper motions are determined for stars in four fields in the Hyades cluster region, each centered on a white dwarf member of the cluster. Reductions of the measures reveal larger errors than is customary for parallaxes found with the Van Vleck refractor. The white dwarfs are much the faintest stars included in the astrometric program, and exposure times were necessarily much longer than usual. This led to fewer plates and exposures than is customary. It also allowed many stars to be measured in each field, with the result that greater flexibility in the reductions and a better understanding of the sources of error are possible.