2 Observations

The observations were obtained in 1991, October 18-23 by JC and in 1992, February 18-24 by WS at the 91 cm Dutch light collector at the European Southern Observatory (ESO), La Silla, Chile. The scale of the f/13.75 telescope is approximately $16.4\hbox{$^{\prime\prime}$ }~{\rm mm}^{-1}$. In both observing runs we used the coated GEC CCD chip ESO #7 with $576 \times 385$ pixels. The pixel size of $22\times 22~\mu {\rm m}^{2}$leads to a theoretical scale of $0.361\hbox{$^{\prime\prime}$ }~{\rm pixel}^{-1}$. The use of filters can alter this value.

The Network team agreed to observe in two wavelength bands: V and I. We took from the filters available in the ESO filter set Bessel V and Gunn i and used neutral density filters when necessary.

Some problems with the focusing of the telescope and camera adapter resulted in a loss of efficiency and a much smaller number of observations per night than expected.

Normally all programme stars were observed at least three times in each filter. We increased the number of exposures up to 20 with increasing brightness ratio of the binary components.

During these runs we observed 288 visual double stars:

-

277 stars from the HIPPARCOS Input Catalogue (HIC, Turon et al. [1992]);

-

11 astrometric standard stars, 4 without HIC number;

-

6 double stars without HIC numbers, identified by their BD number, and

-

HD 214509, one of the photometric standards, which turned out to be a close visual double star.

The programme stars can be classified as "intermediate'' double stars with separations normally in the range $2\hbox{$^{\prime\prime}$ }< \rho < 12\hbox{$^{\prime\prime}$ }$. Four stars have separations $\rho$ less than 2 $^{\prime\prime}$ and 39 stars have separations greater than 12 $^{\prime\prime}$. The programme was limited to magnitude differences between the two components mainly in the range $0\,{\rm mag} < \Delta V < 3\,{\rm mag}$.

The final calibration process was performed on the basis of the HIPPARCOS data (see Sect. 3). As additional tool for defining scale and position angle astrometric standard stars were observed regularly. They have been taken from the lists of Brosche & Sinachopoulos ([1988] and [1989]). In addition, trails of bright stars over the CCD frame were generated by stopping the telescope tracking.

V and I standard stars of the Cousins system with no indications of variability in the Geneva photometry were selected from the lists of Landolt ([1983]), Menzies et al. ([1989]) and Menzies et al. ([1991]) by Grenon (1991, private communication) and measured in each night of good photometric quality.