The systemic velocities of four long-period cataclysmic variable stars

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 (Porb > 5 h) CVs should be typically less than 1.5 Gyr old, and their line-of-sight velocity dispersion (σγ) 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−1 with uncertainties of the order of 3 km s−1, referred to the dynamical local standard of rest, leading to a dispersion of ∼8 km s−1. Calculation of a 95 per cent confidence interval gives the result 4 < σγ < 28 km s−1 compared with a prediction of 15 km s−1. We also have an improved determination of mass donor spectral type, K2 and q for the four systems.