Sample of cataclysmic variables detected in the 400d X-ray survey

A sample of cataclysmic variables (CVs) detected among the X-ray sources of the 400 square degree (400d) survey performed based on ROSAT pointings is presented. A technique for selecting CVs among the X-ray sources using additional optical and infrared data, based on Sloan Digital Sky Survey andWISE data, is described. We present the optical observations of the selected objects carried out mainly with the Russian-Turkish 1.5-m telescope (RTT-150) and the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences (BTA). Some observations have also been performed with the 1.6-m AZT-33IK telescope of the Institute of Solar-Terrestrial Physics, the Siberian branch of the Russian Academy of Sciences. Eight CVs, four of which were detected in our work, have been selected by now. Based on this sample, we have obtained preliminary constraints on the X-ray luminosity function of CVs in the solar neighborhood in the range of low luminosities, L (X) similar to 10(29)-10(30) erg s(-1) (0.5-2 keV). The logarithmic slope of the CV luminosity function in this luminosity range is shown to become gentler than that at L (X) > 10(31) erg s(-1). It follows from our estimates of the CV luminosity function that several thousand CVs will be detected in the SRG all-sky survey at high Galactic latitudes, which will allow much more accurate measurements of their X-ray luminosity function to be obtained.