SIMBAD references

38 optical counterparts of ICRF sources were observed and positions were deduced using USNO A2.0 stars and stars extracted from the preliminary version of the GSC II catalog, named GSC2.2. These positions were compared to the ICRF positions, and to other published positions of the same optical counterparts. A significant distortion of the USNO A2.0 catalog is revealed for stars fainter than the 11th magnitude and with declinations from 0 to 20°. On the other hand, the preliminary version of the GSC II catalog shows a very good external accuracy and a good homogeneity in right ascension and declination for the observed zones.