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

Results of CCD imaging and aperture polarimetry of Comet 21P/Giacobini-Zinn er and Comet C/1989 U5 (Linear) are presented. Observations were carried ou t with the 2-m telescope of Pik Terskol Observatory and with the 0.7-m tele scope of Kharkov State University in November 1998 and January 1999. The ne w observations and published data indicate an anomalous spectral dependence of polarization of Giacobini-Zinner, namely, a negative wavelength gradien t 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 fil ter. 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 overab undance of large particles as compared to the other comets. (C) 2000 Elsevi er Science Ltd. All rights reserved.