SIMBAD references

A set of stable compact radio sources is proposed, based on the analysis of VLBI-derived time series of right ascensions and declinations from mid-1989 through May 2002. Five selection schemes are tested, that are based on the usual and Allan standard deviations and on apparent drifts. The efficiency of the selection schemes is characterized by the ability of the sources to support the maintenance of the direction of the axes of the celestial reference frame that they materialize. When compared with the current set of ICRF defining sources, the best performing selection scheme keeps 199 sources and lowers the medium-term frame instability from 28 to 6 microarcsec.