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 \pm 7 km s ^ { -1 } for the space velocities , CVs in the current refined sample ( 159 systems ) are found to have 5 \pm 1 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 \pm 1 Gyr .
According to 5 \pm 1 and 4 \pm 1 Gyr kinematical ages implied by 52 \pm 8 and 45 \pm 7 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 \pm 5 km s ^ { -1 } and \sigma _ { \gamma } = 26 \pm 4 km s ^ { -1 } .
Small difference of \gamma velocity dispersions between the systems below and above the gap may imply that magnetic braking does not operate in the detached phase , during which the system evolves from the post-common envelope orbit into contact .