CCD photometry and astrometry for visual double and multiple stars of the HIPPARCOS catalogue - III. CCD photometry and differential astrometry for 253 southern "intermediate" systems

We present new astrometric and photometric data for 253 visual double stars of the "intermediate" class, i.e. with angular separations in the range 1 to 15 arcsec. The multi-colour observations were obtained in 1992-94 as part of the ESO Key Programme "Visual Double Stars" performed at La Silla Observatory (Chile). Differential magnitudes and colours have been secured in a systematic way while component magnitudes and colour indices have been determined in the V and I passbands of the Cousins standard system in good photometric conditions only. Internal and mean external errors are smaller than respectively 0.01 mag and a few hundreths of a magnitude (typically less than 0.03 mag). Relative positions are obtained as a by-product with internal errors of the order of 0.006 " in angular separation (corresponding to pixel width x 0.02) and 0.07 degrees in position angle. Final accuracies are thoroughly discussed: we illustrate the overall excellent quality of the data and estimate the quality degradation for angular separations barely larger than the width of the seeing disk.