The parallax of the Pleiades has been mired in controversy ever since the very first astrometric measures in the late 1880s. Over a century later, the measures from the HIPPARCOS catalogue gave results which were inconsistent with the distance inferred from the fitting of the colour-magnitude diagram. We briefly review here the debate and focus on the various attempts made at solving the problem, and especially those using binary stars. The only double-lined eclipsing binary found so far in the Pleiades, HD 23642, provides not only the final answer to the problem but also, through detailed state-of-the-art analyses, the fundamental calibration for binaries in more distant clusters and hence in the Local Group. We discuss some of the various sources of systematic uncertainties that limit, so far, the accuracy of the measured stellar parameters to about 1% and the progress that is required to break this barrier.

The use of eccentric eclipsing binaries to test stellar internal structure models, as well as the equations of motion provided by General Relativity, is reviewed. Close to 80 years have elapsed since the first ideas were produced in this field and many results have been obtained since then. It appears that, in general, a good understanding of stellar structure within the main sequence is available while the same level of knowledge can not be claimed beyond the termination age. The equations of general relativity could not be disproved with observational data though some systems cannot still be fully explained. In the near future, the analysis of evolved systems, very low mass stars, the effects of tidal resonances and the presence of third bodies has to be further explored. In addition, the analysis of large data bases obtained by means of extensive photometric surveys will certainly change the picture from an observational point of view.

Although not one red dwarf can be seen with the naked eye, they dominate the solar neighborhood, accounting for no less than 70% of all stars. The large numbers of these wee denizens of the night actually translate into a surprisingly large amount of mass – in fact, more mass is found in M dwarfs than in any other stellar spectral type.

To determine just how important red dwarfs are to the nature of the Universe, an accurate mass-luminosity relation (MLR) must be determined so that a relatively easily determined characteristic, luminosity, can be converted into the critical parameter, mass. Results from a decade-long observational effort to calibrate the MLR for red dwarfs using the Fine Guidance Sensors (FGSs) on the Hubble Space Telescope (HST) will be highlighted. For many of the binary systems targeted, the interferometric data from HST are combined with radial velocity data to further improve the mass measurements, which often have errors less than 5%. Related results from a large southern sky parallax program to determine accurate distances to red dwarfs, and a search for companions orbiting them, will also be discussed.

The state of mass determinations for M dwarfs' smaller cousins, the L and T dwarfs, will also be reviewed. Although we do not yet know the size of the true population of these (primarily) substellar objects, only by mapping out the interplay of their masses and luminosities (which change drastically with time) can we understand their contribution to the mass budget of the Universe.

Loosely bound, fragile binary stars, whose separations may reach ∼ 0.1 pc, are like open clusters with two coeval components. They provide a largely overlooked avenue for the investigation of many astrophysical questions. For example, the orbital distribution of fragile binaries with two long-lived main-sequence components provides a sensitive test of the cumulative effects of the Galactic environment. In pairs where one component is evolved, the orbits have been amplified by post-main-sequence mass loss, potentially providing useful constraints on the initial-to-final mass relation for white dwarfs. The nearly featureless spectra of cool white dwarfs usually provide little information about intrinsic radial velocity, full space motion, population membership, metallicity, etc. However, distant main sequence companions provide benchmarks against which those properties can be determined. In addition, the cooling ages of white dwarf components provide useful limits on the ages of their main sequence companions, independent of other stellar age determination methods. This paper summarizes some of the ways fragile binaries provide useful leverage on these and other problems of interest.

Considerable effort has been spent to determine the period of tidal circularization in close binaries as a function of age, in order to constrain the tidal dissipation theory. A new, direct method of measuring the tidal dissipation by precise timings of periastron passages in eccentric binaries undergoing circularization is proposed. Such binaries with components just leaving the Main Sequence can be found as inner systems in multiple stars. Three examples are given.

High-resolution spectroscopy of U Gem was obtained during quiescence. We did not find a hot spot or gas stream around the outer boundaries of the accretion disk. Instead, we detected a strong narrow emission near the location of the secondary star. We measured the radial velocity curve from the wings of the double-peaked Hα emission line, and obtained a semi-amplitude value in excellent agreement with the ultraviolet results by Long & Gilliland (1999). We present also a new method to obtain K2, which enhances the detection of absorption or emission features arising in the late-type companion. Our results are compared with published values derived from the near-infrared NaI line doublet. From a comparison of the TiO band with those of late type M stars, we find that a best fit is obtained for a M6 V star, contributing 5% of the total light at that spectral region. Assuming that the radial velocity semi-amplitudes reflect accurately the motion of the binary components, then from our results: Kem = 108 ± 2 km s−1; Kabs = 310 ± 5 km s−1, and using the inclination angle by Zhang & Robinson (1987); i = 69°.7 ± 0.7, the system parameters become: MWD = 1.20 ± 0.05 M⊙; MRD = 0.42 ± 0.04 M⊙; and a = 1.55 ± 0.02 R⊙. Based on the separation of the double emission peaks, we calculate an outer disk radius of Rout/a ∼ 0.63, close to the distance of the inner Lagrangian point L1/a ∼ 0.63. Therefore we suggest that, at the time of observations, the accretion disk was filling the Roche-Lobe of the primary, and that the matter leaving the L1 point was colliding with the disc directly, producing the hot spot at this location. Specific details not included in the printed version can be found in the Electronic Poster (EP).

The runaway O-type stars HD 14633 and HD 15137 are both SB1 systems that were probably ejected from the open cluster NGC 654. Were these stars dynamically ejected by close gravitational encounters in the dense cluster, or did the binaries each receive a kick from a supernova in one member? We present new results from our investigation of the optical, X-ray, and radio properties of these binary systems to discuss the probable ejection scenarios. We argue that these binaries may have been ejected via dynamical interactions in the dense cluster environment.

Wide binaries, particularly in large numbers and as free from selection biases as possible, constitute a largely overlooked tool for studying the Galaxy. The goal of this review is to highlight the potential inherent to large samples of field wide binaries for research on problems as varied as star formation in the early Galaxy, the nature of halo dark matter, the evolution of the stellar halo, new geometric distances, metallicities, masses, and ages of field stars and white dwarfs, and much more. Using the Revised NLTT as an illustrative example, I review the main steps in the assembly of a large catalog of wide binaries useful for multiple applications. The capability of cleanly separating between the Galactic disk and halo populations using good colors and proper motions is emphasized. The critical role of large surveys for research on wide binaries as well as for the better understanding of the Galaxy in general is stressed throughout. Finally, I point out the potential for assembling new samples of wide binaries from available proper-motion surveys, and report on current efforts of using the SDSS towards this goal.

I argue that binary star statistics offer the best observational constraints on current hydrodynamical simulations of star forming clusters. In these simulations, clusters form hierarchically from the bottom up, and dynamical interactions, mediated by the presence of circumstellar material, play a vital role at the lowest (few body) level of the hierarchy. Such a scenario produces a rich array of complex multiple systems whose properties are in many respects consistent with observations. I however highlight two areas of current disagreement: the simulations over-produce low mass single stars and under-produce binaries with low mass ratios. It is currently unclear to what extent these shortcomings reflect numerical issues and to what extent the omission of relevant physical processes. I conclude with a theorist's wish list for observational diagnostics that would most meaningfully constrain future modeling efforts.

We consider the multiplicity of stellar systems with (combined) magnitude brighter than 6.00 in Hipparcos magnitudes. We identify 4555 such bright systems, and the frequencies of multiplicities 1, 2, . . , 7 are found to be 2722, 1412, 299, 86, 22, 12 and 2. We also consider the distributions of periods of orbits and sub-orbits. For the even more restricted set of 474 systems with VH ≤ 4.00 the proportions of higher multiples up to sextuple are progressively larger, suggesting incompleteness in even the relatively well-studied larger sample.

We construct a Monte-Carlo algorithm that will generate systems with roughly the observed multiplicities and orbital parameters, taking account of selection effects.

The study of the flow structure is of great importance, and the results can be used both for consideration of the evolutionary status of binary stars and for the interpretation of observational data. In this report we present the review of 3D gas dynamic models used for the description of the mass exchange in close binaries.

Main features of the flow structure in steady-state close binaries are summarized. It is shown that in self-consistent considerations the interaction between the stream from the inner Lagrangian point and the forming accretion disk is shock-free, and, hence, a “hot spot” does not form at the outer edge of the disk. To explain the presence of the observed zones of high luminosity in close binaries a self-consistent “hot line” model was proposed according to which the excess energy is released in a shock wave formed due to interaction between the circumdisk halo and the stream. The “hot line” model was confronted with observations and confirmed by virtue of comparison of synthetic and observational light curves for cataclysmic variables and by the analysis of Doppler tomograms.

The special attention is paid to the physics of accretion disks in binary systems and particularly to waves in disks. The possible observational manifestations of the “hot line” wave and two arms of the tidal shocks are discussed. We also suggest that an additional spiral density wave can exist in inner parts of the cold accretion disk. This spiral wave is due to the retrograde precession of flow lines in the binary system. The results of 3D gas dynamic simulation have shown that a considerable increase in the accretion rate (by an order of magnitude) is associated with the formation of the “precessional” spiral wave. Based on this fact we suggest a new mechanism for the superoutbursts and superhumps in close binaries.

Observations of young low-mass binaries (t ≲ 107 yr, M ≲ 3 M_⊙) can be used to calibrate pre-Main Sequence (pre-MS) evolutionary tracks. Recent high angular resolution HST/FGS, speckle, and long-baseline interferometry have resolved the astrometric orbits of a few SB2 pre-MS binaries and have provided the individual dynamical masses of their components as well as the system orbital parallaxes. Spectroscopic fits and filter photometry have permitted to determine SpT (temperatures) and a good estimate of the absolute magnitude (bolometric luminosity) of the components, which in turn allows one to place the components on a theoretical HR-diagram. In this way, one can check (a) whether the measured dynamical masses agree with the predicted masses on the tracks and (b) whether both components lie on an isochrone, as they should for a coeval physical pair of stars.

With a sufficiently large sample of different masses and ages of resolved SB2 systems, most of the parameter space of pre-MS tracks can be tested, even for very low stellar masses (M < 0.5 M_⊙) and very young ages (< 2 Myr). This is a prerequisite in order to derive the IMF and star formation history in very young clusters and associations.

We have analysed publicly available MACHO observations of 6833 variable stars in the Large Magellanic Cloud, classified as eclipsing binaries. After finding that a significant fraction of the sample was misclassified, we redetermined periods and variability class for all stars, producing a clean sample of 3031 eclipsing binaries. We have investigated their distribution in the period-color-luminosity space, which was used, for example, to assign a foreground probability to every object and establish new period-luminosity relations to selected types of eclipsing stars. We found that the orbital period distribution of LMC binaries is very similar to those of the SMC and the Milky Way. We have also determined the rate of period change for every star using the O-C method, discovering about 40 eclipsing binaries with apsidal motion, 45 systems with cyclic period changes and about 80 stars with parabolic O-C diagrams. In a few objects we discovered gradual amplitude variation, which can be explained by changes in the orbital inclination caused by a perturbing third body in the system.

We have studied white dwarfs in common proper motion pairs (CPMPs) to improve the semi-empirical initial–final mass relationship of white dwarfs. In this contribution, we report new results obtained from spectroscopic observations of both members of several CPMPs composed of an F, G or K type star and a DA white dwarf.

Understanding the galaxy in which we live is one of the great intellectual challenges facing modern science. With the advent of high quality observational data, the chemical evolution modeling of our galaxy has been the subject of numerous studies in the last years. However, all these studies have one missing element which is ‘the evolution of close binaries’. Reason: their evolution is very complex and single stars only perhaps can do the job. (Un)Fortunately at present we know that a significant fraction of the observed intermediate mass and massive stars are members of a binary or multiple system and that certain objects can only be formed through binary evolution. Therefore galactic studies that do not account for close binaries may be far from realistic. We implemented a detailed binary population in a galactic chemical evolutionary model. Notice that this is not something simple like replacing chemical yields. Here we discuss three topics: the effect of binaries on the evolution of 14N, the evolution of the type Ia supernova rate and the effects on the G-dwarf distribution, the link between the evolution of the r-process elements and double neutron star mergers (candidates of short gamma-ray burst objects).

Wide or fragile pairs are sensitive probes of the galactic potential, and they have been used to provide information about the galactic tidal field, the density of GMC and the masses of dark matter perturbers present in both the disk and the halo. Halo wide binaries and moving clusters, since they are likely to be the remains of past mergers or of dissolved clusters, can provide information on the dynamical and merger history of our Galaxy. Such remnants should continue to show similar motions over times of the order of their ages. We have looked for phase space groupings among the low-metallicity stars of Schuster et al (2006) and have identified a number of candidate moving clusters. In several of the moving clusters we found a wide CPM binary already identified in our catalogue of wide binaries among high-velocity and metal-poor stars (Allen et al 2000a). Spectroscopic follow-up studies of these stars would confirm the physical reality of the groups, as well as allow us to distinguish whether their progenitors are dissolved clusters or accreted extragalactic systems.

The eccentricities of catalogued binaries show that tidal interactions extend to periods of at least 1000 days. The maximum periods for complete circularization probably depend on age, rather than mass. For the longest periods all eccentricities are equally probable. For shorter periods the highest eccentricities disappear first. For the same periods, the SB2's have larger mean eccentricities than the SB1's, in accord with Keplers Third Law.

The frequency distribution f(a) of semimajor axes of double and multiple systems, and their eccentricities and mass ratios, contain valuable fossil information about the process of star formation and the dynamical history of the systems. In order to advance in the understanding of these questions, we made an extensive analysis of the frequency distribution f(a) for wide binaries (a > 25 AU) in various published catalogues, as well as in our own (Poveda et al 1994; Allen et al 2000; (Poveda & Hernández–Alcántara 2003). Based upon all these studies we have established that the frequency distribution f(a) is a function of the age of the system and follows Öpik's distribution f(a) ∼ 1/a in the range of 100 AU < a < ac(t), where ac (t) are the critical semimajor axes beyond which binaries have been dissociated by encounters with massive objects. We argue that the physics behind the distribution f(a) ∼ 1/a is a process of energy relaxation, analogous to those present in stellar clusters (secular relaxation) or in the early stages of spherical galaxies (violent relaxation). The existence of runaway stars indicates that both types of relaxation are important in the process of binary and multiple star dynamical evolution.