**8. Multiplicity of LBVs**

In recent years it became clear, that a significant part of massive stars are born in double (or multiple) systems. A detailed analysis of the results from the FLAMES-Tarantula survey lead Sana et al. [114] to the following percentages for massive stars: effective single (real single stars or wide binaries without significant interaction ∼29%, stellar merger ∼24%, accretion and spin up or common envelope evolution ∼14%, and envelope stripping ∼33% [114]. Therefore, about ∼71% are affected by binary interaction. Alternatively, if one sees the result of mergers as apparent single stars for most of their lifetime, then only ∼47% of the massive stars should show a companion. The same data also imply that the numbers of equal mass binaries are lower than unequal mass pairs. The ratio goes up to ∼50% at M2/M1 = 0.3, the lowest mass ratio probed by their data [114].

This immediately implies that a sizable number of LBVs should have binary companions. The idea, that binary star evolution is linked to the LBV phenomenon is quite old, see e.g., Gallagher (1989) [115]. More recently, the idea of mergers triggering giant eruptions (or being one path to SN impostors) gained some interest, e.g., [116]. Still, the observation of binary companions of LBVs are difficult, due to the large luminosity of the primary, and its strong stellar wind, which both limits spectroscopic searches. Direct imaging searches only cover relatively large separations, and only few LBVs are analyzed with stellar interferometers, yet. A search for X-ray only covers situations in which colliding winds can occur, and may be in part contaminated by the X-ray emission of circumstellar nebula.

The current state on observed stellar companions to LBVs is the following: As shown in Section 4.4 *η* Carinae show strong signs of being a binary star with a massive, hot companion stars. HD 5980 was first reported as an exlipsing LBV Wolf-Rayet binary system that showed an LBV like eruption [117]. Koenigsberger et al. [118] report new analysis which is consistent with the system being more complex and multiple: a double binary scenario and manifests a quadruple system. The LBV candidate [KMN95] Star A (= 1806-20) showes double He lines [119] but single emission lines, implying a dense stellar wind for the primary, similar to the case of *η* Carinae. MCW 314 shows clear indications in its lightcurve and its radial velocity curve for having a lower luminosity supergiant companion [120]. If the wide companion candidate [121] is truly bound, than MWC 314 would be a hierarchical triple star. The LBV HR Car was observed with stellar interferometry and strong indications of a companion was found [122]. The companion star appears to be relatively low mass (below <sup>∼</sup>15M).

A search for wide companions based on natural seeing, AO assisted imaging, and archival HST imaging of 7 galactic LBVs, LBV candidates, and some related objects yielded one star with potential companion (MWC 314) and no apparent bound companions for the 5 other LBVs and LBV candidates (the Pistol star, HD 168625, HD 168607, MWC 930, and [KMN95] Star A (= 1806-20)) [121]. The PSF

subtracted HST images used in the study of LBV nebulae in the LMC by Weis [52] also showed no apparent companion stars, but only relatively large projected orbital distances could be probed (>0.1 pc).

A X-ray archival survey (using XMM-Newton and CHANDRA X-ray satellites) of 31 LBVs, LBV candidates, and related objects was performed by Naze et al. [123]. X-ray emission may indicated colliding winds in a binary, but (softer) X-ray could also be created in a circumstellar nebula, see e.g., Weis [94] for the case of *η* Carinae. The survey of Naze et al. yielded 4 detection (*η* Carinae, W243 (= Westerlund 1 #243), MSX6C G026.4700+00.0207 (= GAL 026.47+00.02), and Schulte #12 (= Cyg OB2 #12). Two more are labeled doubtful candidates (GCIRS 34W, and GCIRS 33SE) by the authors. This result also implies a long list of 25 non-detections, which includes confirmed LBVs like P Cygni, the Pistol star, and FMM 362. While acknowledging their rather heterogenous data base, the authors sugges<sup>t</sup> that their detection rate is consistent with a binary fraction between 26% and 69%, roughly consistent with that of other classes of hot, massive stars.

Given the very different methods used, and the therefore very different orbital radii and mass (and luminosity) ratios probed up to now, it is hard to derive a reliable result on the binary fraction for LBVs as a class. An additional problem are the very different LBV input lists used in the different searches. There are clearly several good cases for binary companions of LBV stars. Still, we regard the actual binary fraction of LBVs as currently very uncertain, but most likely around ∼20% for the confirmed LBVs. This would be somewhat lower than the binary fraction for other classes of massive stars like O supergiants or Wolf-Rayet stars. If this estimate of the binary fraction is correct, it may hold important clues for the evolutionary pathways leading to LBVs.

### **9. LBV and Their Neighborhood**

Smith & Tombleson[124] analyzed the location of LBVs in comparison to their surrounding and concluded that LBVs in MW and LMC are isolated, and not spatially associated with young O-type stars. This would imply a complete change of the standard view of the evolution of LBVs, clearly a far reaching claim, which needed further investigation. Humphreys et al. [125] analyzed the location of a sample of LBVs in M 31, M 33, and the LMC in comparison too other massive main sequence and supergiant stars. With this large and more coherently selected sample,Humphreys et al. [125] concluded that LBVs are associated with supergiant stars and are neither isolated or preferentially run-away stars. Separating the more massive classical and the less luminous LBVs, the classical LBVs have a distribution similar to the late O-type stars, while the less luminous LBVs have a distribution like the red supergiants. Smith [126] questioned the results of this analysis and reiterated the results of his analysis. Davidson et al. [127] shortly after showed that the statistical analysis methods use in [126] are flawed. Independently, Aadland et al. [128] performed a very similar analysis and came to similar conclusions as Humphreys et al. [125], that the stellar environment of LBVs is the same as for supergiants. It is still be worth noting, that the Aadland et al. sample is not a clean LBV sample, but contains many B[e] supergiants. Note that this point was also pointed out by Kraus in her review paper on B[e] in this volume. In a recent paper Smith [129] gravitated to the interpretation by Humphreys et al. of LBV locations within (or near) their birth association. Just lately with an analysis of GAIA data [130], strong evidence was presented, that OB stars form not preferentially in bound clusters, but in a continuous distribution of gas densities, at many locations of the birth cloud. This view is also supported by recent simulations which also favor a hierarchical formation model for the formation of OB stars as a result of the fractal structure of the birth clouds, contrary to a monolithic collapse. In this picture many different stellar neighborhoods of massive stars would be natural, also consistent with our results.
