  The secondary in cataclysmic variables (CV's) 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 co-adding spectra, called skew mapping, has
been proposed in the past. Gradually, examples of its application are being
published. Nonetheless, 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 method's power over techniques that are more
conventional, when the signal-to-noise (s/n) 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 K2=127+-23 km/s
for SY Cnc, K2=144+-18 km/s for RW Sex, and K2=262+-14 km/s for UX UMa. These
we believe to be the first direct determinations of K2 for these objects.
Furthermore, we also obtain K2=263+-30 km/s 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 Ser, for which we propose more tentative values of K2.
Clear failures of the method are also discussed (EF Eri, VV Pup, SW Sex).
