Speckle observations with PISCO in Merate: VIII. Astrometric measurements of visual binaries in 2007 and new orbits of the multiple system Zeta Aqr

We present relative astrometric measurements of visual binaries made during the second semester of 2007, 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. We obtained 283 new measurements of 279 objects, with angular separations in the range 0″.17–4″.4, and an average accuracy of 0″.014. The mean error on the position angles is 0°.6. Most of the position angles were determined without the usual 180° ambiguity with the application of triple-correlation techniques and/or by inspection of the long integration files. We also present the new orbit we have computed for Zeta Aqr AB (ADS 15971), for which our measurements lead to large residuals with the previously computed orbit. We were also able to compute the elements of the perturbation orbit Bb-P caused by an invisible companion, whose mass is estimated at 0.7 M⊙ (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)