We have provided a set of 681 new secondary standard stars which are useful for the transformation of instrumental UBVRI-CCD data to the standard Johnson-Cousins system. These stars cover a wide interval in visual magnitude as well as in colour indexes. They are distributed in 11 different fields, all of them located around Landolt (1983, 1992) primary standards and mainly in the interval from 5 to 8 hours in right ascension.
Acknowledgements
The 1.23 m telescope is operated by the Max-Planck Institut für Astronomie at Centro Astronómico Hispano-Alemán, Calar Alto, Almería, Spain. The 1.52 m telescope is operated by Observatorio Astronómico Nacional at Calar Alto, Almería, Spain. This work was supported by the CICYT under contract ESP95-0180. D.G.-E. acknowledges partial support from Área de Formación del Instituto Nacional de Técnica Aeroespacial "Esteban Terradas'' (Becas de Formación en Astrobiología). The authors thank Dr. Leonel Gutiérrez (UNAM) for his comments on CCD detectors.
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