Corrections of proper motions in declination by using ILS data

There are nowadays numerous astrometric ground{based observations of some stars referred to Hipparcos Catalogue, made at many observatories during the last century. We used the data on latitude variations, covering the period 1899.7 - 1979.0, of visual Zenith Telescopes (ZT) of International Latitude Service (ILS), to improve the Hipparcos proper motions in declination for stars observed at seven ILS stations: Carloforte, Cincinnati, Gaithersburg Kitab, Mizusawa, Tschardjui and Ukiah. About 15 years elapsed since the HIPPARCOS ESA mission (ESA 1997) observations (1991.25 is the epoch of this catalogue), and with the errors of the Hipparcos proper motions close to 1 mas/yr, the error of apparent places of stars is now more than 15 mas; so that it exceeds the error of the Hipparcos positions by one order of magnitude (which is about 1 mas). Also, for some Hipparcos stars, the errors of proper motions are much larger than the averaged value itself, even not realistic at all (Vondr?k et al. 1998); the Hipparcos astronomical satellite mission lasted less than four years, not enough to get a sufficient accuracy of the proper motions. To improve the accuracy of the proper motions for some Hipparcos stars, the ground-based data were used and some new catalogues were published (such as ARIHIP, EOC-2, etc) during the last decade. Our investigations are in accordance with the Earth Orientation Catalogue - EOC (Vondr?k and Ron 2003) one, based on the Earth rotation programmes ground{based data, but we used different method here. Our results yield better proper motions in declination for stars common to ILS and HIPPARCOS and a good agreement with those from EOC-2.