Context. The nearby high-mass star binary system θ 1 Ori C is the brightest and most massive of the Trapezium OB stars at the core of the Orion Nebula Cluster, and it represents a perfect laboratory to determine the fundamental parameters of young hot stars and to constrain the distance of the Orion Trapezium Cluster. Aims. By tracing the orbital motion of the θ 1 Ori C components, we aim to refine the dynamical orbit of this important binary system. Methods. Between January 2007 and March 2008, we observed θ 1 Ori C with VLTI/AMBER near-infrared (H-and K-band) longbaseline interferometry, as well as with bispectrum speckle interferometry with the ESO 3.6 m and the BTA 6 m telescopes (Band V -band). Combining AMBER data taken with three different 3-telescope array configurations, we reconstructed the first VLTI/AMBER closure-phase aperture synthesis image, showing the θ 1 Ori C system with a resolution of ∼2 mas. To extract the astrometric data from our spectrally dispersed AMBER data, we employed a new algorithm, which fits the wavelength-differential visibility and closure phase modulations along the H-and K-band and is insensitive to calibration errors induced, for instance, by changing atmospheric conditions. Results. Our new astrometric measurements show that the companion has nearly completed one orbital revolution since its discovery in 1997. The derived orbital elements imply a short-period (P ≈ 11.3 yr) and high-eccentricity orbit (e ≈ 0.6) with periastron passage around 2002.6. The new orbit is consistent with recently published radial velocity measurements, from which we can also derive the first direct constraints on the mass ratio of the binary components. We employ various methods to derive the system mass (M system = 44 ± 7 M ) and the dynamical distance (d = 410 ± 20 pc), which is in remarkably good agreement with recently published trigonometric parallax measurements obtained with radio interferometry.

Orbital binary stars are essential objects for determining dynamical and physical properties of stars through a combined analysis of photometric and astrometric data. We compiled a set of orbital binaries with known trigonometric parallaxes and orbits of high quality, using data from current versions of the Catalogue of Orbits and Ephemerides of Visual Double Stars and the Sixth Catalog of Orbits of Visual Binary Stars as well as including updated information from WDS and SIMBAD. We constructed distributions of orbital binaries among dynamical mass, period, semi-major axis, and eccentricity of systems, which characterize the set. Some problems related to the parameterization of orbital binaries are also discussed in the paper.

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Abstract. The scientific preparation of the Gaia mission encompasses both the data reduction algorithms and the generation of simulated data which have to be as realistic as possible. In this respect, binaries and multiple stars are a mandatory component in the simulation of the Milky Way as they impact on the performance tests of the on-ground data processing. The ingredients for the simulation of multiple stars are described and the predictions are compared to observed data or outcomes from stellar multiplicity studies.

Spectral observations of the flare star EV Lac made on the 2.6-m telescope at the Byurakan Observatory in 2000-2002 are reported. A powerful flare was detected and it was possible to follow the variation in the equivalent widths of the Hα and Hβ lines, both during the flare and during the quiescent phase of the star. It is shown that the profiles and equivalent widths of the lines underwent substantial changes. The equivalent widths of these emission lines reached a maximum roughly 40 min after the flare maximum.