This is an era of enormous opportunity in binary star research. The Leuven meeting provided an assessment of the broad scope of observational work, merits of the analysis methods, and connections with theory. Here I outline some of the issues related to spectroscopic, photometric, and high angular resolution observations of binary stars.

Energy generation by nuclear fusion is the fundamental process that prevents stars from collapsing under their own gravity. Fusion in the core of a star converts hydrogen to heavier elements from helium to uranium. The signature of this nucleosynthesis is often visible in a single star only for a very short time, for example while the star is a red giant or, in massive stars, when it explodes. Contrarily, in a binary system nuclear-processed matter can captured by a secondary star which remains chemically polluted long after its more massive companion star has evolved and died. By probing old, low-mass stars we gain vital insight into the complex nucleosynthesis that occurred when our Galaxy was much younger than it is today. Stellar evolution itself is also affected by the presence of a companion star. Thermonuclear novae and type Ia supernovae result from mass transfer in binary stars, but big questions still surround the nature of their progenitors. Stars may even merge and one of the challenges for the future of stellar astrophysics is to quantitatively understand what happens in such extreme systems. Binary stars offer unique insights into stellar, galactic and extragalactic astrophysics through their plethora of exciting phenomena. Understanding the chemical evolution of binary stars is thus of high priority in modern astrophysics.

The VLT-FLAMES Tarantula Survey is an ESO Large Program from which we have obtained multi-epoch optical spectroscopy of over 800 massive stars in the 30 Doradus region of the Large Magellanic Cloud. This unprecedented dataset is being used to address outstanding questions in how massive stars evolve from the early main sequence to their deaths as core collapse supernovae. Here we focus on the rotation properties of the population of presumably single O stars and use binary population synthesis predictions to show that the rapid rotators among this population likely are post-interaction binaries. The same type of population synthesis can be used to study the mass function of massive young clusters. We argue – on the basis of predictions for the Arches and Quintuplet clusters – that a sizable fraction of the very massive WNh stars in 30 Doradus may also have such a binary interaction history. We single out the WNh star discovered in the VFTS, VFTS 682, and discuss its properties.

The complexity of composite spectra of close binary star system makes study of the spectra of their component stars extremely difficult. For this reason there exists very little information on the photospheric chemical composition of stars in close binaries, despite its importance for informing our understanding of the evolutionary processes of stars. In a long-term observational project we aim to fill this gap with systematic abundance studies for the variety of binary systems. The core of our analysis is the spectral disentangling technique, which allows isolation of the individual component star spectra from the time-series of observed spectra. We present new results for high-mass stars in close binaries. So far, we have measured detailed abundances for 22 stars in a dozen detached binary systems. The parameter space for the stars in our sample comprises masses in the range 8–22 M⊙, surface gravities of 3.1–4.2 (c.g.s.) and projected rotational velocities of 30–240 km s−1. Whilst recent evolutionary models for rotating single stars predict changes in photospheric abundances even during the main sequence lifetime, no star in our sample shows signs of these predicted changes. It is clear that other effects prevail in the chemical evolution of components in binary stars even at the beginning of their evolution.

We discuss recent progress made in the spectral modelling of OB stars from the main sequence to evolved phases as BA-type supergiants. Non-LTE line-formation computations can now reproduce observed spectra over the entire optical and near-IR wavelength range with high confidence, facilitating stellar atmospheric parameters and elemental abundances to be determined at high accuracy and precision. An overview is given how the fundamental stellar parameters of single stars determined in our new approach compare to high-precision data derived from detached eclipsing massive binary stars. Finally, the observational constraints for a sample of Galactic objects are put in context with state-of-the-art evolution models for rotating massive stars.

Precise determinations of effective temperatures and surface gravities are mandatory to derive not only reliable chemical abundances, but also parameters like distances, masses, radii and luminosities in OB stars. We have previously determined atmospheric parameters of 30 well-studied OB main sequence to giant stars via multiple ionization equilibria in NLTE, reaching uncertainties as low as ∼300 K for effective temperature and ∼0.05 dex for surface gravity. Based on the comparison of our spectroscopic parameters and reddening-independent quantities of the Johnson and Strömgren photometric systems, temperature and gravity calibrations are proposed to different photometric indices depending on the luminosity class. With these calibrations, effective temperatures can be determined at a precision of ∼400 K for luminosity classes III/IV and of ∼800 K for luminosity class V. Surface gravities can reach internal uncertainties as low as ∼0.08 dex. Our uncertainties are smaller than typical differences among other methods in the literature, which reach values up to ± 2000 K for temperature and ± 0.25 dex for gravity, and in extreme cases, + 6000 K and ± 0.4 dex, respectively.

bonnfires, a new generation of population synthesis code, can calculate nuclear reaction, various mixing processes and binary interaction in a timely fashion. We use this new population synthesis code to study the interplay between binary mass transfer and rotation. We aim to compare theoretical models with observations, in particular the surface nitrogen abundance and rotational velocity. Preliminary results show binary interactions may explain the formation of nitrogen-rich slow rotators and nitrogen-poor fast rotators, but more work needs to be done to estimate whether the observed frequencies of those stars can be matched.

Massive stars feature highly energetic stellar winds that interact whenever two such stars are bound in a binary system. The signatures of these interactions are nowadays found over a wide range of wavelengths, including the radio domain, the optical band, as well as X-rays and even γ-rays. A proper understanding of these effects is thus important to derive the fundamental parameters of the components of massive binaries from spectroscopic and photometric observations.

In the last five years our knowledge of magnetism in early-type stars has significantly improved because of the new gen- eration of high-resolution spectropolarimeters (ESPaDOnS@CFHT, Narval@TBL, HARPSpol@ESO). The success of the Magnetism in Massive Stars (MiMeS) Project, for example, has greatly improved our understanding of the magnetic properties of massive early-type stars; however, it was mainly focused on single stars. Summarized here is our general understanding of stellar magnetism in early-type stars and the recent findings from the MiMeS project. As recent results suggest a significant fraction of early-type stars are in binaries, this implies that the interplay between magnetic fields and binarity has yet to be investigated in any significant detail. To this end, the current analysis of the recently discovered magnetic field in the rapidly-rotating secondary star of the close, massive binary system known as Plaskett’s star – a unique system that is testing our current theories of binarity and magnetism in early-type stars, and a target for investigation in the recently accepted Binarity and Magnetic Interactions in various classes of Stars (BinaMIcS) Large Programs at CFHT and TBL – is discussed.

We present the BinaMIcS project, its goals and the first BinaMIcS spectropolarimetric observations obtained with Narval at TBL and ESPaDOnS at CFHT: dedicated time-series for targeted hot and cool close double-line spectroscopic (SB2) binaries and a survey of hot close SB2 binaries. The very first results are also presented. In particular, our first survey observations seem to show a lack of magnetic fields in hot close binaries compared to single stars.

Cepheids provide approaches to determining binary parameters which are often complementary to those for main sequence massive and intermediate mass stars. Specifically, we are using high resolution imaging, radial velocities, and X-ray studies to determine binary characteristics. Among the results are that they have both a high frequency of binary systems, and also a high proportion of triple systems.

Accurate determinations of masses and radii in binary stars, along with estimates of the effective temperatures, metallicities, and other properties, have long been used to test models of stellar evolution. As might be expected, observational constraints are plentiful for main-sequence stars, although some problems with theory remain even in this regime. Models in other areas of the H-R diagram are considerably less well constrained, or not constrained at all. I summarize the status of the field, and provide examples of how accurate measurements can supply stringent tests of stellar theory, including aspects such as the treatment of convection. I call attention to the apparent failure of current models to match the properties of stars with masses of 1.1–1.7 M⊙ that are near the point of central hydrogen exhaustion, possibly connected with the simplified treatment of convective core overshooting.

Modeling the convection of stellar interiors is crucial for stellar evolutionary studies. During the last decades different approximations (phenomenological, simulations, etc.) have been developed in order to improve the convection description. In spite of that, most of the stellar evolutionary tracks or isochrones are built, basically, using a theory established more than 50 years ago: the famous mixing length theory (MLT). It is very well known the MLT limitations and its inconsistencies. However MLT is still strongly used. Probably for good reasons (e.g. good predictions for the solar model and user friendly implementation in stellar evolutionary codes). The computation of stellar evolutionary models using MLT is dependent, at least, of two free parameters: the MLT parameter (particularly important for the super-adiabatic layer, in convective envelopes) and the amount of overshooting in convective cores. In this framework, the question is: are MLT parameter or overshoting constants from star to star (independently of the stellar mass, chemical composition and evolutionary stage)? To help to answer this question the FGK stars members of binaries play an important role. In this presentation, we point out the main contributions of binaries for the convection parameters estimation, in particularly for the mixing length parameter.

Eclipsing binary systems offer a unique opportunity to accurately measure the fundamental properties of stars. In the past years, masses and radii of low-mass stars in these systems have been obtained with uncertainties well below the 3% level, imposing stringent constrains on theoretical models. Several results revealed that there is a discrepancy between the radius predicted by models and that from the observations of about 5–10% depending on the mass range. On the other hand, effective temperatures are overestimated by models by about a 10%. These discrepancies have been usually attributed to stellar magnetic activity, however, the correct scenario is not yet clear. The progress in the understanding of stellar activity on low-mass stars and its relation with the radius discrepancies with models is summarized here both from the empirical and the theoretical points of view.

We discuss results associated with ongoing surveys to measure the diameters and temperatures of main-sequence stars with interferometry. Empirical data such as these are used to calibrate less direct relationships to extend our knowledge to a large number of stars. The data are also used to identify weaknesses in stellar atmosphere and evolutionary models as well as provide empirical constraints to aid in the development of new models.

Strong photospheric chemical peculiarities are observed in a significant fraction of main sequence stars from B to early F spectral types. The development of these chemical peculiarities is understood in the framework of atomic diffusion. However, the details leading from this theoretical framework to the diversity of observed surface abundance characteristics, particularly the impact of competing processes, are not fully understood. In the chemically peculiar Am and HgMn stars, binarity appears to be a basic ingredient providing the conditions necessary for these peculiarities to develop. In contrast, the magnetic chemically peculiar Ap and Bp stars appear to be anomalously rare in short period binary systems. This likely provides a clue to the origin of the magnetic fields in Ap and Bp stars.

We present results obtained using the magnetic Dartmouth stellar evolution code that address the possibility that magnetic fields are inflating low-mass stars in detached eclipsing binaries. While it seems plausible that magnetic fields are inflating stars with radiative cores, the level of inflation observed among fully convective stars appears too large to be explained by magnetic fields. We provide an alternative explanation, stellar metallicity, and propose observations that can help further constrain stellar models.

W UMa-type contact binaries are composed of two late-type main-sequence stars, where both components are filling their critical Roche Lobes and sharing a common convective envelope. Their formation and evolution are unsolved problems in stellar astrophysics. This kind of binary systems have the lowest angular momentum and shortest orbital periods among main-sequence binaries. One of the possibilities for their origin is that the angular momentums of binary stars are transferred from the central binaries to close-in companions. In this paper, we will summarize some of our recent progresses on searching for close-in companions to contact binary stars, including the closest stellar companion to a contact binary at an orbital separation of about 0.8 AU. Then, based on the observational properties of those close-in companions, the formation and the evolution of contact binaries will be discussed.

CEMP-s stars are very metal-poor stars with enhanced abundances of carbon and s-process elements. They form a significant proportion of the very metal-poor stars in the Galactic halo and are mostly observed in binary systems. This suggests that the observed chemical anomalies are due to mass accretion in the past from an asymptotic giant branch (AGB) star. Because CEMP-s stars have hardly evolved since their formation, the study of their observed abundances provides a way to probe our models of AGB nucleosynthesis at low metallicity. To this end we included in our binary evolution model the results of the latest models of AGB nucleosynthesis and we simulated a grid of 100 000 binary stars at metallicity Z = 0.0001 in a wide range of initial masses and separations. We compared our modelled stars with a sample of 60 CEMP-s stars from the SAGA database of metal-poor stars. For each observed CEMP-s star of the sample we found the modelled star that reproduces best the observed abundances. The result of this comparison is that we are able to reproduce simultaneously the observed abundance of the elements affected by AGB nucleosynthesis (e.g. C, Mg, s-elements) for about 60% of the stars in the sample.