  Nearly 50 post-common-envelope (post-CE) close binary central stars of
planetary nebulae (CSPNe) are now known. Most contain either main sequence or
white dwarf (WD) companions that orbit the WD primary in around 0.1-1.0 days.
Only PN~G222.8-04.2 and NGC~5189 have post-CE CSPNe with a Wolf-Rayet star
primary (denoted [WR]), the low-mass analogues of massive Wolf-Rayet stars. It
is not well understood how H-deficient [WR] CSPNe form, even though they are
relatively common, appearing in over 100 PNe. The discovery and
characterisation of post-CE [WR] CSPNe is essential to determine whether
proposed binary formation scenarios are feasible to explain this enigmatic
class of stars. The existence of post-CE [WR] binaries alone suggests binary
mergers are not necessarily a pathway to form [WR] stars. Here we give an
overview of the initial results of a radial velocity monitoring programme of
[WR] CSPNe to search for new binaries. We discuss the motivation for the survey
and the associated strong selection effects. The mass functions determined for
PN~G222.8-04.2 and NGC~5189, together with literature photometric variability
data of other [WR] CSPNe, suggest that of the post-CE [WR] CSPNe yet to be
found, most will have WD or subdwarf O/B-type companions in wider orbits than
typical post-CE CSPNe (several days or months c.f. less than a day).
