Modelling Population II Cataclysmic Variables

We study the long--term evolution of cataclysmic variables (CVs) as a function of the companion star metallicity. It is shown that CVs with secondaries of low metallicity (Z=10 \Gamma4 ) which belong to population II (PopII CVs) pass through a detached phase with a smaller period width, a shorter minimum period, and display a slightly higher mass transfer rate, resulting in shorter evolutionary timescales compared to CVs with a solar chemical composition of the secondary. A population synthesis model for PopII CVs with a star formation period lasting for 10 9 yr after the onset of Galaxy formation shows that most PopII CVs have already evolved beyond the minimum period. As these systems are faint, they are difficult to observe. Extracting a magnitude--limited sample from the computed intrinsic models and taking into account the sample's incompleteness towards fainter apparent magnitudes we derive a period distribution both for PopII CVs and the more common population I CVs. From a ..