The properties of cataclysmic variables in photometric H alpha surveys

We report on the properties of 71 known cataclysmic variables (CVs) in photometric H alpha emission-line surveys. Our study is motivated by the fact that the Isaac Newton Telescope (INT) Photometric H alpha Survey of the northern galactic plane (IPHAS) will soon provide r', i' and narrow-band H alpha measurements down to r' similar or equal to 20 for all northern objects between - 5 degrees < b < +5 degrees. IPHAS thus provides a unique resource, both for studying the emission-line properties of known CVs and for constructing a new CV sample selected solely on the basis of H alpha excess. Our goal here is to carry out the first task and prepare the way for the second. In order to achieve this, we analyse data on 19 CVs already contained in the IPHAS data base and supplement this with identical observations of 52 CVs outside the galactic plane. Our key results are as follows: (i) the recovery rate of known CVs as H alpha emitters in a survey like IPHAS is similar or equal to 70 per cent; (ii) of the similar or equal to 30 per cent of CVs which were not recovered similar or equal to 75 per cent were clearly detected but did not exhibit a significant H alpha excess at the time of our observations; (iii) the recovery rate depends only weakly on CV type; (iv) the recovery rate depends only weakly on orbital period; (v) short-period dwarf novae tend to have the strongest H alpha lines. These results imply that photometric emission-line searches provide an efficient way of constructing CV samples that are not biased against detection of intrinsically faint, short-period systems