Photographic zenith tubes observations to improve HIPPARCOS proper motion in declination of some stars

A procedure of calculation and results of an attempt to improve proper motions in declination for some HIPPARCOS stars are presented. A long series of optical observations of these stars observed with Photographic Zenith Tubes (PZT) are used. The HIPPARCOS ESA mission (ESA 1997) was nearly 4 years long, but each of the different Earth rotation programmes (PZTs included) covers a few decades in the interval 1899.7 1992.0 (Vondrák et al. 1998). The HIPPARCOS mission period of a few years only is not enough to determine the precise proper motions for some stars, with the standard error of proper motions of about 1 mas/year, but groundbased data covering a few decades with positional errors one or two orders higher than HIPPARCOS ones are nevertheless useful for checking or improving some HIPPARCOS proper motions (mostly of double and multiple stars). Here, the observations of two PZTs (with different longitudes and the same latitude) are used to improve some HIPPARCOS proper motions in declination; the method is explained and some results are presented.