AM Herculis systems are binary stars with mass transfer onto a highly magnetic white dwarf. The mass flow is funneled by the magnetic field towards the polar caps of the white dwarf. The shocked plasma above the polar cap provides a unique laboratory to study the properties of high energy plasma in strong magnetic fields. We are involved in a study of quasi-periodic oscillations in this polar cap plasma. In 1998 we observed a couple of sources with RXTE and made simultaneous optical observations with 2.3m National Australian telescope at Siding Springs.