2002MNRAS.337.1215N

Although a large number of orbital periods of cataclysmic variable (CV) stars have been measured, comparison of period and luminosity distributions with evolutionary theory is affected by strong selection effects. A test has been discovered that is independent of these selection effects and is based upon the kinematics of CVs. If the standard models of evolution are correct then long-period (P_orb> 5 h) CVs should be typically less than 1.5 Gyr old, and their line-of-sight velocity dispersion (σ_γ ) should be small. We present results from a pilot study, which indicate that this postulate is indeed true. Four long-period dwarf novae (EM Cyg, V426 Oph, SS Cyg and AH Her) were observed over a complete orbit, in order for accurate radial velocities to be obtained. We find values of -1.7, 5.4, 15.4 and 1.8 km/s with uncertainties of the order of 3 km/s, referred to the dynamical local standard of rest, leading to a dispersion of ∼8 km/s. Calculation of a 95 per cent confidence interval gives the result 4 < σ_γ< 28 km/s compared with a prediction of 15 km/s . We also have an improved determination of mass donor spectral type, K₂ and q for the four systems.