The age of cataclysmic variables: a kinematical study
  Using available astrometric and radial velocity data, the space velocities of
cataclysmic variables (CVs) with respect to Sun were computed and kinematical
properties of various sub-groups of CVs were investigated. Although
observational errors of systemic velocities ($\gamma$) are high, propagated
errors are usually less than computed dispersions. According to the analysis of
propagated uncertainties on the computed space velocities, available sample is
refined by removing the systems with the largest propagated uncertainties so
that the reliability of the space velocity dispersions was improved. Having a
dispersion of $51\pm7$ km s$^{-1}$ for the space velocities, CVs in the current
refined sample (159 systems) are found to have $5\pm1$ Gyr mean kinematical
age. After removing magnetic systems from the sample, it is found that
non-magnetic CVs (134 systems) have a mean kinematical age of $4\pm1$ Gyr.
According to $5\pm1$ and $4\pm1$ Gyr kinematical ages implied by $52\pm8$ and
$45\pm7$ km s$^{-1}$ dispersions for non-magnetic systems below and above the
period gap, CVs below the period gap are older than systems above the gap,
which is a result in agreement with the standard evolution theory of CVs. Age
difference between the systems below and above the gap is smaller than that
expected from the standard theory, indicating a similarity of the angular
momentum loss time scales in systems with low-mass and high-mass secondary
stars. Assuming an isotropic distribution, $\gamma$ velocity dispersions of
non-magnetic CVs below and above the period gap are calculated
$\sigma_\gamma=30\pm5$ km s$^{-1}$ and $\sigma_\gamma=26\pm4$ km s$^{-1}$.
