A radial velocity survey for post-common-envelope Wolf-Rayet central stars of planetary nebulae: first results and discovery of the close binary nucleus of NGC 5189

The formation of Wolf-Rayet central stars of planetary nebulae ([WR] CSPNe) whose spectroscopic appearance mimics massive WR stars remains poorly understood. Least understood is the nature and frequency of binary companions to [WR] CSPNe that may explain their H-deficiency. We have conducted a systematic radial velocity (RV) study of six [WR] CSPNe to search for post-common-envelope (post-CE) [WR] binaries. We used a cross-correlation method to construct the RV time series as successfully done for massive close binary WR stars. No significant RV variability was detected for the late-[WC] type nuclei of Hen 2-113, Hen 3-1333, PMR 2 and Hen 2-99. Significant, large-amplitude variability was found in the [WC4] nucleus of NGC 5315. In the [WO1] nucleus of NGC 5189, we discovered significant periodic variability that reveals a close binary with P_orb = 4.04 ± 0.1 d. We measured a semi-amplitude of 62.3 ± 1.3 km s^-1 that gives a companion mass of m₂ ≥ 0.5 M_⊙ or m₂ = 0.84 M_⊙ (assuming i = 45°). The most plausible companion type is a massive white dwarf (WD) as found in Fleming 1. The spectacular nebular morphology of NGC 5189 fits the pattern of recently discovered post-CE PNe extremely well with its dominant low-ionization structures (e.g. as in NGC 6326) and collimated outflows (e.g. as in Fleming 1). The long 4.04 d orbital period is either anomalous (e.g. NGC 2346) or it may indicate that there is a sizeable population of [WR] binaries with massive WD companions in relatively wide orbits, perhaps influenced by interactions with the strong [WR] wind.