1997A&A...320..136S

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^–4) 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 10⁹yr 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 differential comparison of the models we calculate the probability that a CV at a given vector distance r from the Sun is member of the population II. Based on our results, we find that a reasonable observable sample of PopII CVs can only be found for z>2000pc.