2. Observations and reductions
The photometric data have been gathered during three campaigns at La Silla, Chile. For the A-component HD 220392, 7 nights of measurements were made in June 1990, 11 nights were obtained in September 1991 and 3 in October 1992. For the B-component HD 220391, only observations made in September 1991 and October 1992 are available. The June 1990 and September 1991 campaigns have been performed by P. Lampens with the Swiss 0.7m telescope of the Geneva Observatory while the October 1992 data were obtained with the ESO 0.5m telescope by D. Sinachopoulos. We have collected a total of 396 data for the brightest component HD 220392 and 245 for HD 220391. The characteristics of these data are mentioned in Table 1. Standard and additional programme stars have also been observed during these nights. All Geneva measurements are absolute measurements in the filters UBVB1B2V1G obtained through a centralized reduction scheme at the Geneva observatory (Rufener 1988). This centralized Swiss processing has not been applied to the ESO data taken in the UBV photometric system. The reduction of the October data implied using a check-star HD 220729 [F4V, V=5.52, B-V=+0.40] whose measurements were interpolated between the two other ones. We have verified the constancy of this star in the Hipparcos catalogue (mag,mag) and we have fitted a 5th degree polynomial to the check-star data for each night separately. Then we have subtracted this polynomial from the data of both programme stars in order to suppress as well as possible common variations. The ESO data are thus being interpreted as differential measurements relative to HD 220729 only. We kept the differential data acquired at the end of the nights at relatively large airmasses ( 1.6) though they are affected by larger noise, after some trials with various combinations. Our results will thus be based on the largest available datasets. In addition we made use of the data provided in the Hipparcos Epoch Photometry Catalogue (ESA 1997).
© European Southern Observatory (ESO) 2000
Online publication: April 17, 2000