An analysis of X-ray and optical light curves of the magnetic cataclysmic variable (MCV) BY Cam is presented. This system is one of three MCVs in which the spin period of the white dwarf and the binary orbital period differ by --1 per cent. As such these `BY Cam' stars are important objects with which to probe the field structure of the magnetic white dwarf and ultimately the nature of synchronization of AM Her binaries. We confirm asynchronous rotation of the magnetic white dwarf with respect to the binary. We find evidence that the accretion stream accretes directly on to the white dwarf as in AM Her systems, but further, the stream impacts on to different magnetic poles over the course of the beat period. We present evidence that the optical and hard X-ray light curves modulate in phase, but together they are out of phase with the soft X-ray light curve. We confirm the spin down of the white dwarf which is expected to lead to the synchronization of the spin and orbital periods of BY Cam.

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Abstract. We present results of imaging and aperture polarimetry of the dust of comet 2P/Encke at phase angles [91][92][93][94][95][96][97][98][99][100][101][102][103][104][105] • , obtained during the 2003 apparition. We investigate how strongly molecular emissions transmitted by the filters used in the observations can affect the resulting polarization of cometary dust. This problem is of particular importance for so-called gasrich comets like comet 2P/Encke which has particularly strong molecular emission as compared to its dust continuum. Aperture polarimetry in the wide-band UBVR filters was performed at the 2.6-m Shain telescope and 1.25-m telescope of the Crimean Astrophysical Observatory on November 17-24. From these measurements a dust polarization of ≈8% is derived, which puts the comet in the class of comets with low polarization. The imaging observations of comet 2P/Encke were carried out at the 2-m telescope of the Bulgarian National Astronomical Observatory on November 20-22, 2003. Narrow-band filters centered on the 0-7-0 transition of the à 2 A 1 -X 2 B 1 electronic band system of NH 2 (662 nm) and on an adjacent red continuum at 642 nm were employed. The polarization of NH 2 averaged over the 0-7-0 vibronic transition amounts to ≈7% at phase angles close 90• , similar to the polarization of the two-atomic molecules CN and C 2 . The dust polarization however, when corrected for the effect of molecular emissions, is larger than 30%. We conclude that the division of comets into two polarimetric classes with one class having in the visual wavelength range a maximum polarization less than 20% is caused by ignoring the contribution of molecular emission and therefore is an artifact. Whether the comet displays a strong silicate feature (i.e. its dust grains are small) or not, the dust polarization is high.

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