UBVRI POLARIMETRY OF AM HERCULIS-TYPE BINARIES .5. THE ASYNCHRONOUS (QUESTIONABLE) POLAR BY CAMELOPARDALIS (H0538+608)

The basic features of the light and circular polarization curves of BY Cam in 1990 Sep can be reproduced by a model with the inclination i = 40-60-degrees of the magnetic white dwarf spin axis and two extended emission regions: a strong positive circular polarization region at th e colatitude beta = 30-45-degrees and a weaker negative circular polar ization region at beta = 100-125-degrees with a phase lag of about 0.5 . The degree of linear polarization was small (less than or similar to 1 %), but the observed position angle pattern is roughly consistent w ith the above geometric picture. However, in November 1990 the reduced amplitude of the position angle curve, and the self-eclipsing-type li ght curves with distinct bright and faint phases, suggest higher incli nation (i approximately 85-degrees). Precession with a period of 100-1 50 days is offered as a hypothetical mechanism for changing the inclin ation. Timings for the transit of the positive circular polarization r egion from 1992 Nov and 1993 Jan appear approximately 0.3 phase later than predicted from our linear ephemeris based on data from the time i nterval JD2446138-8155, indicating a possible period increase of dP/dt approximately 3.3 10(-9), and a synchronization time scale of approxi mately 1200 y. This is longer than found for the only well established resynchronizing magnetic binary V1500 Cyg (approximately 185 y). Furt her timings are needed to confirm and improve the rotational period of the magnetic white dwarf in BY Cam.