tokens: of each other. Finally, we compare a total system luminosity of log ( L /L ⊙ ) = 6.38 (log ( L 1 /L ⊙ ) ≈ 6.25 and log ( L 2 /L ⊙ ) ≈ 5.8) to the tracks for Galactic metallicity from Ekström et al. ( 2012 ), which go up to 120 M ⊙ . While this comparison is much less accurate than the bonnsai method, these luminosities correspond to masses of approximately M 1 ≈ 100 M ⊙ and M 2 ≈ 60 M ⊙ at the ZAMS. This is in excellent agreement with our results from the LMC tracks. 4.3 Origin and evolutionary state Due to its projected distance of 16 arcmin from the massive Westerlund 2 cluster, it has been suggested that WR21a originates from this cluster. Roman-Lopes et al. ( 2011 ) proposed that WR21a might have been dynamically ejected from the Westerlund 2 core, together with the massive binaries WR20aa and WR20c. These stars are in a similar geometrical configuration in the plane of the sky as AE Aur, μ Col, and ι Ori, for which a binary ejection scenario has been proposed (Blaauw 1961 ). Disruption of the orbit by dynamical interactions may explain the high eccentricity of the WR21a system, which is higher than any other massive binary with similar period (Sana et al. 2012 ). The systematic RV of WR21a that follows from our orbital solution is comparable to that of Westerlund 2 (Rauw, Sana Nazé 2011 ), and thus there is no indication for a significant runaway motion in the line of sight if the system indeed originates from this cluster. At the maximum distance estimate of 8 kpc, the projected distance of WR21a from Westerlund 2 is 37 pc. WR21a would have to travel at a speed of 36 km s −1 along the plane of the sky in order to reach its current position within 1 Myr. This is well within the maximum velocity at which a massive binary can be ejected (Perets Šubr 2012 ). The derived spectral types of the components of WR21a can be used to estimate their evolutionary state. Hydrogen-rich WN stars are very massive main sequence stars that are so luminous that their winds are optically thick (de Koter et al. 1997 ). They are located close to the ZAMS in the Hertzsprung–Russell diagram (e.g. Hamann, Gräfener Liermann 2006 ), and thus the primary O3/WN5ha star is expected to be very young, likely around 1–2 Myr. The O3 secondary star is likely of similar age, as stars this massive will evolve towards later spectral types within 2 Myr unless they are rotating rapidly (e.g. Brott et al. 2011 ). The full width at half-maximum of He ii λ4541 Å (3.0206 ± 0.0996 Å) indicates a rotational velocity of v rot sin i ≈ 66 km s −1 following the Ramírez-Agudelo et al. ( 2015 ) calibration ( v rot = 77 km s −1 for an inclination i = 588). Thus, we have no indication for such rapid rotation. 5 SUMMARY AND CONCLUSIONS We have presented multi-epoch observations of the massive binary system WR21a, including a full coverage of the 2011 January periastron passage. The data allow us to obtain orbital solutions for both components of WR21a. We derive very accurate minimum masses
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