A spectroscopic search for faint secondaries in cataclysmic variables

The secondary in cataclysmic variables (CVs) is usually detected by cross-correlation of the CV spectrum with that of a K or M dwarf template, to produce a radial velocity curve. Although this method has demonstrated its power, it has its limits in the case of noisy spectra, such as are found when the secondary is faint. A method of coadding spectra, called skew mapping, has been proposed in the past. Gradually, examples of its application are being published; none the less, so far no journal article has described the technique in detail. To answer this need, this paper explores in detail the capabilities of skew mapping when determining the amplitude of the radial velocity for faint secondaries. It demonstrates the power of the method over techniques that are more conventional, when the signal-to-noise ratio is poor. The paper suggests an approach to assessing the quality of results. This leads in the case of the investigated objects to a first tier of results, where we find K-2 = 127 +/- 23 km s(-1) for SY Cnc, K-2 = 144 +/- 18 km s(-1) for RW Sex and K-2 = 262 +/- 14 km s(-1) for UX UMa. These we believe to be the first direct determinations of K-2 for these objects. Furthermore, we also obtain K-2 = 263 +/- 30 km s(-1) for RW Tri, close to a skew mapping result obtained elsewhere. In the first three cases, we use these results to derive the mass of the white dwarf companion. A second tier of results includes UU Aqr, EX Hya and LX Set, for which we propose more tentative values of K-2. Clear failures of the method are also discussed (EF Eri, VV Pup and SW Sex).