The space density of cataclysmic variables: constraints from the ROSAT North Ecliptic Pole Survey

We use the ROSAT North Ecliptic Pole (NEP) survey to construct a small, but purely X-ray flux-limited sample of cataclysmic variable stars (CVs). The sample includes only 4 systems, 2 of which (RX J1715.6+6856 and RX J1831.7+6511) are new discoveries. We present time-resolved spectroscopy of the new CVs and measure orbital periods of 1.64 \pm 0.02 h and 4.01\pm 0.03 h for RX 1715.6+6856 and RX J1831.7+6511, respectively. We also estimate distances for all the CVs in our sample, based mainly on their apparent brightness in the infrared. The space density of the CV population represented by our small sample is (1.1 +2.3/-0.7) 10^-5 pc^-3. We can also place upper limits on the space density of any sub-population of CVs too faint to be included in the NEP survey. In particular, we show that if the overall space density of CVs is as high as 2 10^-4 pc^-3 (as has been predicted theoretically), the vast majority of CVs must be fainter than L_X \simeq 2 10^29 erg/s.