1997A&A...324..109M

We present the results of fast temporal optical spectroscopy and photometry of the AM Her type system BY Cam obtained in February 1990 and March 1991. Emission line profiles show a complex structure and are strongly variable. The radial velocity studies of a sharp component detected in Balmer lines in March confirms the non-synchronism of the system. Different possible emitting regions are discussed to explain the characteristics of the multiple line components. It is shown from a computation of the line velocities and widths that the 'horizontal' stream cannot explain the lines of intermediate widths observed in this system. A critical review of the different estimations of the periods found in BY Cam is presented. Additional periods are revealed from a re-analysis of previous optical polarimetric and UV spectroscopic data. We show that because of the slight asynchronism between the white dwarf rotation and the orbital period, significant changes in the accretion geometry 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 dipole rotates in the orbital frame with a period of 14.5±1.5 days.