It is great privilege for me to welcome so many of you to Prague, or Praha as we call our capital in Czech.

We have been witnessing a very rapid development of observational techniques over the past few decades, leading to many exciting discoveries, quite often in the field of extragalactic research. I sometimes sense the tendency to consider the studies of binaries and stars in general as exhausted topics. However, we all know that the better our real and deep understanding of the principal sources of radiative energy in the Universe becomes, the safer will be the ground for studies of all higher hierarchical systems.

An overview is presented of the many new and exciting developments in binary and multiple star studies that were discussed at IAU Symposium 240. Impacts on binary and multiple star studies from new technologies, techniques, instruments, missions and theory are highlighted. It is crucial to study binary and multiple stars because the vast majority of stars (>60%) in our Galaxy and in other galaxies consist, not of single stars, but of double and multiple star systems. To understand galaxies we need to understand stars, but since most are members of binary and multiple star systems, we need to study and understand binary stars. The major advances in technology, instrumentation, computers, and theory have revolutionized what we know (and also don't know) about binary and multiple star systems. Data now available from interferometry (with milliarcsecond [mas] and sub-mas precisions), high-precision radial velocities (∼1-2 m/s) and high precision photometry (<1–2 milli-mag) as well as the wealth of new data that are pouring in from panoramic optical and infrared surveys (e.g., > 10,000 new binaries found since 1995), have led to a renaissance in binary star and multiple star studies. For example, advances have lead to the discovery of new classes of binary systems with planet and brown dwarf components (over 200 systems). Also, extremely valuable data about binary stars are available across the entire electromagnetic spectrum — from gamma-ray to IR space missions and from the ground using increasingly more powerful and plentiful optical and radio telescopes as well as robotic telescopes. In the immediate future, spectral coverage could even be extended beyond the radio to the first detection of gravity waves from interacting close binaries. Also, both the quality and quantity of data now available on binary and multiple stars are making it possible to gain unprecedented new insights into the structure, and formation and evolution of binary stars, as well as providing valuable astrophysical information (like precise stellar masses, radii, ages, luminosities and distances) to test and constrain current astrophysical theory. These major advances permit tests of current theories and ideas in stellar astrophysics and provide the foundations for the next steps in modeling and improvements in theory to be taken.

In keeping with its co-sponsorship by members of both the “close” and “wide” binary star communities, IAU Symposium 240 has been jointly dedicated to the honor of Czech astronomer Mirek J. Plavec and the memory of U.S. astronomer Charles E. Worley.

In keeping with its co-sponsorship by members of both the “close” and “wide” binary star communities, IAU Symposium 240 has been jointly dedicated to the honor of Czech astronomer Mirek J. Plavec and the memory of U.S. astronomer Charles E. Worley.

Charles Worley, long-time astronomer at the U.S. Naval Observatory, was born on May 22, 1935, in Iowa City, Iowa, and grew up in Des Moines the son of an M.D., Charles L. Worley, and his wife Iona Cooney Worley, a homemaker. He became interested in astronomy at age nine. His first observational work as an amateur astronomer was plotting and recording more than 10,000 meteors for the American Meteor Society. Continuing his love for astronomy he attended Swarthmore College, where he took part in the parallax program as an Observing Assistant. He also met the other love of his life, his wife, Jane Piper. They were married in 1956 next to Sproul Observatory on the Swarthmore campus. He obtained a B.A. in mathematics from San Jose State College in 1959. He worked for the Lick Observatory in California (1959–1961) as a Senior Assistant and Research Astronomer under a Naval Research grant to observe double stars. After arriving at the U.S. Naval Observatory in 1961, he was the motive force behind an extensive program of double star observation (being himself, a prolific observer), instrumental innovation, and double star cataloging. He quickly gained recognition as one of the world's leading experts in the field of double star astronomy. Charles died on New Year's Eve, 1997, two days before his scheduled retirement.

Long-baseline optical interferometry (LBI) can nearly close the gap in selection space between astrometric and spectroscopic detection of binary star systems, bringing the complementary powers of astrometry and spectroscopy to bear on a complete dynamical understanding of such systems, particularly including the determination of the masses of the individual stellar components. In the case of double-lined spectroscopic systems, their resolution by long-baseline interferometry also yields the orbital parallax and hence the luminosities of the individual stars. In some of these cases, the angular diameters of one or more components are accessible, and so a complete specification of a star in terms of its mass, radius and luminosity is made.

The northern hemisphere is now equipped with several interferometers of unprecedented capability in terms of their baseline sizes, numbers of telescopes and telescope apertures. These instruments, most notably the Palomar Testbed Interferometer at Mt. Palomar Observatory, have produced very significant results of a number of interesting systems fulfilling interferometry's promise to produce fundamental astrophysical data at levels of accuracy that challenge or confirm astrophysical theory.

This paper presents basic principles of long-baseline interferometric study of binary stars and summarizes results from northern interferometers with specific examples of their broad impact on binary star astronomy.

The first multiple-aperture interferometric study of a binary system, in which the power of combining interferometric and spectroscopic data was demonstrated, was made from the Southern Hemisphere. The observations of α Vir with the Narrabri Stellar Intensity Interferometer (NSII) were combined with spectroscopic and photometric data to yield the mass, radius and luminosity of the primary as well as an accurate distance to the system. The NSII also revealed a number of stars, previously thought to be single, to be binary systems. Several of these systems have subsequently been shown to be spectroscopic binaries.

The Sydney University Stellar Interferometer (SUSI) and the European Southern Observatory's Very Large Telescope Interferometer (VLTI) are the two current Southern Hemisphere multiple aperture interferometers. SUSI is being used to determine interferometric orbits for some of the binary systems discovered with the NSII including β Cen and λ Sco and, in combination with spectroscopy, to determine accurate masses for early-type stars and accurate dynamical parallaxes for the systems.

The VLTI has operated with three beam-combining instruments, namely VINCI, MIDI and AMBER. The few observations of binary systems that have been made so far are summarised and, while in general they are of a preliminary nature, they demonstrate the potential of the VLTI for binary star studies.

One double-lined spectroscopic binary that has been observed with all three Southern Hemisphere instruments is γ2 Vel, which has the brightest Wolf-Rayet star in the sky as its secondary. The observations and preliminary results for the masses of the O-type primary and WC8 secondary and for the distance to the system are summarised.

The SIM (Planet Quest) mission is a space-based long-baseline stellar interferometer designed for ultra-precise astrometry. This paper describes how SIM can be used for double star research. There are several regimes of operation. For binary stars separated by more than 1″.5, SIM treats these as distinct objects. Double stars less than ∼10 milliarcsec in separation are seen as a single object and SIM measures the photocenter of the composite object. Between 10 mas and 1″.15, SIM is able to see the double star as two distinct objects, but because photons from both stars are detected there is the possibility of increased noise and measurement bias. This paper describes how double stars are observed with SIM and what information can be derived.

We present a progress report on a program to improve the orbits of known spectroscopic binaries that are potential targets for ground-based optical interferometers, such as the CHARA array and PTI. The combination of such observations results in three-dimensional orbits that produce very accurate masses (uncertainties of less than 0.5%) as well as orbital parallaxes that are generally more precise than those from Hipparcos. After additional analyses determine other basic parameters of the stars, the components can be compared with theoretical evolutionary tracks. We are currently using three observatories to obtain high-resolution, red-wavelength spectra of 44 systems. Results from four years of observation are highlighted. Interferometric observations of some systems are in progress.

The advent of larger telescopes and powerful instrumentation enables the exploration of new aspects of faint eclipsing binaries that are just now becoming accessible. An example of this are eclipsing binaries in Local Group galaxies such as the LMC, SMC, M31 and M33, whose study yields not only stellar properties of stars formed in different chemical environments (thus providing useful model tests) but also direct distance determinations to the host galaxies. In general this is also applicable to eclipsing binaries belonging to any stellar ensemble. Another example is the observation and study of eclipsing very-low mass stars, brown dwarfs and planets. Besides the need for large telescopes because of their faintness, these also benefit from improved observational capabilities in the infrared spectral windows. Here we discuss the prospects for eclipsing binary research using photometry and spectroscopy from large telescopes.

In the last decade, over 7000 eclipsing binaries have been discovered in the Local Group through various variability surveys. Measuring fundamental parameters of these eclipsing binaries has become feasible with 8 meter class telescopes, making it possible to use eclipsing binaries as distance indicators. Distances with eclipsing binaries provide an independent method for calibrating the extragalactic distance scale and thus determining the Hubble constant. This method has been used for determining distances to eclipsing binaries in the Magellanic Clouds and the Andromeda Galaxy and most recently to a detached eclipsing binary in the Triangulum Galaxy by the DIRECT Project. The increasing number of eclipsing binaries found by microlensing and variability surveys also provide a rich database for advancing our understanding of star formation and evolution.

Changes in the double star database are highlighted, describing various methods of observation (both historically and those of the past few years) and their effectiveness in different regimes of separation space. The various niches for wide- and narrow-field work as they apply to double and multiple stars are examined and the different types of information which each can provide are described. Despite the significant growth of the double star database, much can still be done, such as finding lost pairs, filling in missing parameters so that observing programs can select all stars appropriate to their capabilities, or providing at least gross kinematic descriptions. After more than 20 years of successful work, speckle interferometry and conventional CCD astrometry have replaced filar micrometry and photography as preferred classical techniques. Indeed, most work in filar micrometry is now being done by amateurs. Work on pairs described as neglected in the last major WDS data release (2001) is given as a specific example. Finally, the continued need to publish data in classical double star parameters is also discussed.

Due to improvements in technology (interferometers, precision radial-velocity techniques, etc.), the traditional separation/period regimes of “wide” and “close” binaries are witnessing increasing overlap. This is expected to lead to increasing confusion in component identification, since different observing techniques adopted their own rules for designation of these components. A quick overview will be given of the Washington Multiplicity Catalog, an effort to create a common nomenclature scheme for all types of stellar and sub-stellar companions.

Polaris, the nearest and brightest classical Cepheid, is a member of at least a triple system. It has a wide (18″) physical companion, the F-type dwarf Polaris B. Polaris itself is a single-lined spectroscopic binary with an orbital period of ∼30 years (Kamper 1996). By combining Hipparcos measurements of the instantaneous proper motion with long-term measurements and the Kamper radial-velocity orbit, Wielen et al (2000) have predicted the astrometric orbit of the close companion. Using the Hubble Space Telescope and the Advanced Camera for Surveys' High-Resolution Channel with an ultraviolet (F220W) filter, we have now directly detected the close companion. Based on the Wielen et al orbit, the Hipparcos parallax, and our measurement of the separation (0″.176 ± 0″.002), we find a preliminary mass of 5.0 ± 1.5 M⊙ for the Cepheid and 1.38 ± 0.61 M⊙ for the close companion. These values will be refined by additional HST observations scheduled for the next 3 years.

We have also obtained a Chandra ACIS-I image of the Polaris field. Two distant companions C and D are not X-rays sources and hence are not young enough to be physical companions of the Cepheid. There is one additional stellar X-ray source in the field, located 253″ from Polaris A, which is a possible companion. Further investigation of such a distant companion is valuable to confirm the full extent of the system.

DE CVn is a relatively unstudied eclipsing binary where one of the components is an M dwarf and the other is a white dwarf. Its brightness makes it an ideal system for a detailed study in the context of common-envelope evolution of a detached white dwarf – red dwarf binary with a relatively short orbital period (∼8.7 hours). We present a detailed study of the basic parameters (e.g. orbital period, components' masses and spectral types) for this system from photometric and spectroscopic studies. The eclipses observed during several photometric observing runs were used to derive the ephemeris. We have used spectroscopic data to derive the radial velocity variations of the emission lines and these are used to determine the components' masses and the orbital separation. The secondary component in DE CVn is an M3 main-sequence star and the primary star, which only contributes to the blue continuum, is a cool white dwarf with a temperature of ∼8000 K. From the photometry and spectroscopy together, we have set a limit on the binary inclination. This system is a post-common-envelope system where the progenitor of the present day white dwarf was a low-mass star (M≤2M⊙). The time before DE CVn becomes a semi-detached system is longer than the Hubble time.

We have redetermined the absolute dimensions of the mid B-type eclipsing binary U Oph from new light and radial-velocity curves, accounting for both the apsidal motion and the light-time orbit around the third star. The stars in U Oph have masses of 5.27 and 4.74 M⊙(±1.5%) and are located in the middle of the main-sequence band for an an age of ∼50 Myr. U Oph and three other systems (V760 Sco, MU, Cas and DI Her) all have components within 10% of 5M⊙ and ages below 100 Myr; we find significant heavy-element abundance differences between these young nearby stars.

The method of spectra disentangling has been applied in many studies of different stellar systems up to the distance of Andromeda Galaxy. In some of these applications the underlying assumptions are not precisely satisfied. This is why new generalizations of the method are needed. Ways to overcome these problems are discussed here.

We report on a high-spatial-resolution survey for binary stars in the periphery of the Orion Nebula Cluster, at 5–15 arcmin (0.65 – 2 pc) from the cluster center. We observed 228 stars with adaptive optics systems, in order to find companions at separations of 0.13 – 1.12 arcsec (60 – 500 AU), and detected 13 new binaries. Combined with the results of Petr (1998), we have a sample of 275 objects, about half of which have masses from the literature and high probabilities to be cluster members. We used an improved method to derive the completeness limits of the observations, which takes into account the elongated point spread function of stars at relatively large distances from the adaptive optics guide star. The multiplicity of stars with masses >2 M⊙ is found to be significantly larger than that of low-mass stars. The companion star frequency of low-mass stars is comparable to that of main-sequence M-dwarfs, less than half that of solar-type main-sequence stars, and 3.5 to 5 times lower than in the Taurus-Auriga and Scorpius-Centaurus star-forming regions. We find the binary frequency of low-mass stars in the periphery of the cluster to be the same or only slightly higher than for stars in the cluster core (< 3′ from θ1C Ori). This is in contrast to the prediction of the theory that the low binary frequency in the cluster is caused by the disruption of binaries due to dynamical interactions. There are two ways out of this dilemma: Either the initial binary frequency in the Orion Nebula Cluster was lower than in Taurus-Auriga, or the Orion Nebula Cluster was originally much denser and dynamically more active. A detailed report of this work has been published in Astronomy & Astrophysics (Köhler et al. 2006).

Techniques to extract information from spectra of unresolved multi-component systems are revised, with emphasis on recent developments and practical aspects. We review the cross-correlation techniques developed to deal with such spectra, discuss the determination of the broadening function and compare techniques to reconstruct component spectra. The recent results obtained by separating or disentangling the component spectra is summarised. An evaluation is made of possible indeterminacies and random and systematic errors in the component spectra.