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.