Speckle observations with PISCO in Merate: IV. Astrometric measurements of visual binaries in 2005

International audienceWe present relative astrometric measurements of visual binaries made during the second semester of 2005, with the speckle camera PISCO at the 102 cm Zeiss telescope of Brera Astronomical Observatory, in Merate. Our sample contains orbital couples as well as binaries whose motion is still uncertain. The purpose of this long term program is to improve the accuracy of the orbits and determine the masses of the components. We performed 130 new observations of 120 objects, with most of the angular separations in the range 0. ′′ 1-4 ′′ , and with an average accuracy of 0. ′′ 01. Most of the position angles could be determined without the usual 180 • ambiguity with the application of triple-correlation techniques, and their mean error is 0. • 8. We have found a possible new triple system: ADS 11077. The measurements of the closest binaries were made with a new data reduction procedure, based on model fitting of the background of the auto-correlations. As this procedure proved to be very efficient, we have reprocessed the old observations of close binaries made with PISCO in Merate since 2004. We thus improved 20 measurements already published and obtained 7 new measurements for observations that were previously reported as "unresolved". We finally present revised orbits for ADS 684, MCA 55Aac (in the Beta 1 Cyg-Albireo multiple system) and ADS 14783 for which the previously published orbits led to large residuals with our measurements and for which the new observations made since their computation allowed a significant improvement of those old orbits. The sum of the masses that we derived for those systems are consistent with the spectral type of the stars and the dynamic parallaxes are in good agreement with the parallaxes measured by Hipparcos