6 Conclusions

We have obtained accurate and homogeneous CCD photometric data of 276 visual double stars of the HIPPARCOS Input Catalogue. The data include Cousins V and I magnitudes and colour indices V-I for the individual double star components as well as the differences between the components. The internal errors of these differences are only 0.005 mag in V band and 0.006 mag in the I band. After an elaborated error analysis, including comparison with joint magnitudes of binary systems measured in different photometric systems (Johnson UBV, Geneva, Strömgren) we arrive at an external error not larger than 0.030 mag.

Angular separations $\rho$ and position angles $\theta$ have been secured for 288 double stars. As a consequence of the repeated measurements (up to 15 or more for increasing brightness ratios of the components), the internal errors are very small: $0.004\hbox{$^{\prime\prime}$ }$in separations, $0.05\hbox{$^\circ$ }$ in position angles and $0.011\hbox{$^{\prime\prime}$ }$ in position differences between measurements in different filters. A preliminary look at the positional differences between the CCD and Hipparcos data indicates very good agreement and the errors on the differential CCD astrometry seem to be as small as the Hipparcos data. This will be quantified in subsequent papers.

Acknowledgements

We thank Patricia Lampens (Brussels) and Edouard Oblak (Besançon) for their assistance in the "Visual Double Stars'' programme and for the many enlightening discussions. JC acknowledges funding by project 2.0060.91 of the Fund for Scientific Research (F.W.O.) - Flanders(Belgium).