No abstract

It is generally accepted that the presence of intinsic polarization of Be stars confirms the basic assumption of an extended non-spherically symmetric envelope, as well as the origin of polarization in scattering of non-polarized stellar radiation on free electrons in the envelope.The long-term polarization variations, for years to decades have been examined in a very limited number of stars. The behaviour of polarization parameters during all activity phases (B, Be, shell ...) is not known with certainty.Having in mind the importance of polarization data of Be stars, on one side, and ability to observe in a very long series with the same telescope available at Belgrade Observatory, on the other, a program of sty dying long-term optical polarization changes has been set up in 1974. The aim was to obtain reliable longterm data on polarization changes in V spectral region and examine these changes during different activity phases of Be stars.Polarimetrie observations at Belgrade Observatory were carried out with the 65-cm Zeiss refractor and the stellar Polarimeter (Kubicela et al. 1976 ) which was modified in 1979 to enable one to obtain digital magnetic records suitable for further computer processing. The measurements were done in the V spectral region. Under "one measurement" we understand up to 8 one-minute Polarimetrie sine-wave signals ρ hase-aver aged. The typical standard deviation of one 8 -minute measurement is 0.07% for Stokes parameters. The interstellar components were estimated or used from the literature. For the star ο And the observed polarization is presented.In Fig. 1 the individual measurements of the observed polarization percentage (a) and corresponding position angles (b) of the star ο And are shown. Figure 2a and b displays the intrinsic polarization percentage and position angles of the star 88 Her. The data for star κ Dra are presented in this Proceedings. The part of 7 Cas data are published in the paper of Arsenijevic et al. (1990). The BU Tau data are published in the article of Arsenijevic et al. (1993).From the sample of our Polarimetrie data of five stars and published data of V magnitude and H a emission strength existing in the literature for the same star, one can come to the followinig conclusion:The intrinsic visual polarization percentage for all five observed stars was smaller than 1 percent. The long-term variations of both intrinsic polarization parameters exsist for all five stars.