Anomalous wavelength dependence of polarization of Comet 21P/Giacobini-Zinner

Results of CCD imaging and aperture polarimetry of Comet 21P/Giacobini-Zinner and Comet C/1989 U5 (Linear) are presented. Observations were carried out with the 2-m telescope of Pik Terskol Observatory and with the 0.7-m telescope of Kharkov State University in November 1998 and January 1999. The new observations and published data indicate an anomalous spectral dependence of polarization of Giacobini-Zinner, namely, a negative wavelength gradient of the degree of polarization. It is shown that this effect is not caused by the presence of molecular emission in the pass band of the employed filter. The uncommon spectral trend of polarization of Comet Giacobini-Zinner may be caused by a large content of organic matter in its dust or an overabundance of large particles as compared to the other comets.