THE CASE OF THE 2-PERIOD POLAR BY CAMELOPARDALIS (H0538+608)

We present the results of fast temporal optical spectroscopy and photo metry of the AM Her type system BY Cam obtained in February 1990 and M arch 1991. Emission line profiles show a complex structure and are str ongly variable. The radial velocity studies of a sharp component detec ted in Balmer lines in March confirms the non-synchronism of the syste m. Different possible emitting regions are discussed to explain the ch aracteristics of the multiple line components. It is shown from a comp utation of the line velocities and widths that the 'horizontal' stream cannot explain the lines of intermediate widths observed in this syst em. A critical review of the different estimations of the periods foun d in BY Cam is presented. Additional periods are revealed from a re-an alysis of previous optical polarimetric and UV spectroscopic data. We show that because of the slight asynchronism between the white dwarf r otation and the orbital period, significant changes in the accretion g eometry introduce a bias in the period determination depending on the length of the observations. Large phase variations are shown to exist which are well reproduced by a phase-drift model in which a magnetic d ipole rotates in the orbital frame with a period of 14.5 +/- 1.5 days.