In this paper we discuss the impact of extragalactic binaries on astrophysics and briefly summarize the oral papers presented in Joint Discussion 13.

Photographic surveys of the Magellanic Clouds and the M31 carried decades ago discovered ~200 close binaries. In most cases the photographic light curves are good enough only to identify the stars as close (mostly eclipsing) binaries and to estimate orbital periods and binary type. Except for a few cases, little useful information may be obtained from these stars except to be reassured that eclipsing binary (EB) systems are present in other galaxies. However a major advance occurred with the advent of high quantum efficient CCDs. Because of this an explosion in the number of known extragalactic binaries occurred during the 1990s as offshoots of photometric microlensing surveys such as EROS, MACHO, and OGLE. Now over 10 000 extragalactic EBs have been identified. Also, photometric surveys of M31 and M33 (e.g. DIRECT) are discovering many more 19-20th mag eclipsing/close binaries. Over the next decade it is expected that ~1 million new binary systems will be identified in these galaxies. In this overview I will discuss recent advances and future expectations in the studies of extragalactic binaries

As an introduction to the stage on which Extragalactic Binaries exist I will present an overview of the stellar populations of the Local Group Galaxies. This will highlight the (few) similarities in the star formation histories of Local Group galaxies as well as the better-known variety that has been uncovered in recent studies. In the few cases where this is possible, the direct role of binaries in the interpretation of the star forming histories of these galaxies will be described. The possible role of binaries in helping to shape the star-formation history will be described more generally with some emphasis on the sorts of observations that can be carried out to investigate this more fully.

The debate about the distance to the Large Magellanic Cloud (LMC) has an epic history full of controversial and dramatic claims (i.e., see review by Walker 2003), and yet in recent years a standard distance modulus has emerged due primarily to the completion of the Hubble Space Telescope (HST) Key Project to measure the Hubble constant (Preedman et al. 2001). The adopted standard distance modulus,µ0 = 18.5±0.1 mag, yields H0 = 71±10 km s−1 Mpc−1 (total error) in excellent agreement with that derived from the Wilkinson Microwave Anisotropy Probe: H0 72±5 km s−1 Mpc−1 (Spergel et al. 2003), which lends considerable support to its accuracy.

The MACHO Collaboration has produced a variable star catalog of the Magellanic Clouds which is a rich source of eclipsing binary systems. I will review the techniques that have been used by MACHO for identifying eclipsing binaries in a relatively automated fashion which is necessitated by the more than 230000 variable light curves cataloged. We have identified about 6000 eclipsing binaries using these techniques. These systems have periods ranging from hours to more than 1000 days. I will survey the binary star data from MACHO and highlight its use in investigating topics spanning the physics of supersoft gamma-ray sources to the distance of the Large Magellanic Cloud and point to future directions of research using this public resource.

We present results of a search for eclipsing binaries in the Magellanic Cloud fields covering central parts of these galaxies. The data were collected during the second phase of the Optical Gravitational Lensing Experiment survey (OGLE-II) in 1997-2000. In total, about 1500 and 3000 eclipsing stars were found in the Small and Large Magellanic Cloud, respectively (Udalski et al. 1998, Wyrzykowski et al. 2003). The photometric data of all objects are available to the astronomical community from the OGLE Internet archive (http://sirius.astrouw.edu.pl/~ogle/). OGLE-II data contain a full variety of classical eclipsing objects of all types: Algol EA-type, β-Lyr EB-type and W UMA EW-type stars. Large samples of stars allow to study in detail statistical properties of eclipsing objects. OGLE data also contain many very unusual eclipsing stars. Examples include eclipsing variable B-type stars (Mennickent et al. 2003), many spotted stars or eclipsing stars with a Cepheid as a component (Udalski et al. 1999). Recently three objects from the LMC revealing simultaneously RR Lyr and eclipsing binary type variability were discovered (Soszyński et al 2003). If the follow-up observations confirm that both components are physically bound and not optical blends these stars will provide a unique opportunity of direct determination of physical parameters of RR Lyr pulsating stars.

The DIRECT project (as in “direct distances”) started in 1996 with the long-term goal of obtaining distances to two important galaxies in the cosmological distance ladder – M31 and M33 – using detached eclipsing binaries (DEBs) and Cepheids. These two nearby galaxies are the stepping stones in most of the current effort to understand the evolving universe at large scales. Not only are they essential to the calibration of the extragalactic distance scale, but they also constrain population synthesis models for early galaxy formation and evolution. However, accurate distances are essential to make these calibrations free from large systematic uncertainties.

Double-lined eclipsing binaries are (still) our main source for accurate information on stellar masses, radii, and luminosities. Also, they offer very direct distance determinations, useful within the Milky Way and for Local Group Galaxies. I will briefly review the methods involved and discuss critically their advantages and limitations. Furthermore, past and recent highlights will be presented.

The OGLE survey of the SMC has discovered ~1500 eclipsing binaries thereby providing an excellent platform to study the evolution of close binary systems through case A and case B mass-exchange processes. The complementary spectroscopic radial-velocity studies of these binaries are now in progress and are revealing many interesting systems which challenge current theoretical models of close binary star evolution. These studies also provide excellent direct determinations of distances to these binary stars leading to an improved understanding of the mean distance to the SMC and its 3-D structure. Comparisons between these binary-star distances and other methods of determining the mean distance to the SMC will also be made.

The SMC I band light curves of the OGLE project show a curious absence of familiar Algols. Comparitive statistical work for SMC and Milky Way Algols might uncover an evolutionary explanation, presumably connected with chemical composition, or may point to observational selection or even merely perception. As background, LMC and SMC abundances of nitrogen, oxygen, and neon are lower than in our Galaxy by respective factors of about 2.5 and 6, raising the issue of metallicity effects on semi-detached (SD) statistics – specifically on SD light curves and numbers of SDs. Lobe overflow is a triggered process, with both system age and structural configuration at triggering dependent on composition.

Contact binary stars of the W UMa-type (also known as W) are unique objects: The luminosity, produced almost exclusively in the more massive component is efficiently distributed through the common envelope so that the surface brightness is practically identical over the whole visible surface of the binary. Mass ratios are known to span the whole wide range, from almost unity to very small values, as small as q = 0.066. Typically, the primary component provides the luminosity, while both components provide the radiating area. The range of the primary masses is moderate and corresponds to Main Sequence spectral types from middle A to early K and roughly maps into the orbital-period range of about 1.5 days to 0.22 days.

Microlensing surveys of the Large and Small Magellanic Clouds have revealed the existence of Type I and II Cepheid variable stars in eclipsing binary systems. In this review I will summarize the state of the known published and unpublished observations of these systems describe what has been learned to date and discuss what the prospects are for extracting additional information from the known systems using future observations. This review will also discuss the known state of searches for RR Lyrae stars in both spectroscopic and eclipsing binaries and suggest strategies for future success in detecting such systems.

We present accurate BV light curves and continuum energy curves of the EROS eclipsing binaries in the Large Magellanic Cloud to find accurate binary parameters as well as their distances. The observations have been carried out using the 2.1 meter telescope in CASLEO Argentina during Feb. 1-10, 2003. We have concentrated CCD direct observations upon EROS field 1 and 2 to improve the accuracy of light curves of eclipsing binaries in the fields. The spectroscopic observations have been also carried out using the simple dispersion method to get continuum energy curves between wavelengths of 4000-8000 Å for several EROS eclipsing binaries. At first, we determined a combined temperature of both components of each binary system using the continuum. Then, the combined temperature was resolved using the first estimation of the light curve solution. Finally, we determined the photometric solutions of several binaries in the Large Magellanic Cloud.

Analysis of the radial-velocity and light curves of detached eclipsing binaries allows to derive stellar masses and radii and, in consequence, enables to find their distances. The method has been already applied to several LMC binaries, but in order to have the distance to the LMC determined with good accuracy, the parameters need to be known for a larger number of systems. As a first step we present results of the analysis of the photometry of over eighty detached binaries in the LMC selected from the OGLE-II catalog of 53 000 variable star candidates. If possible, we combine the OGLE-II data with the photometry from other projects (MACHO and EROS). As a result, we present the list of brightest eclipsing binaries in the LMC suitable for distance determination.

We present results of our ongoing observing program on search and studies of massive stars (O and WR type) in binary systems in our neighbor galaxies, the Magellanic Clouds. Radial velocity orbits are presented for two new binaries, one in the Small Magellanic Cloud and another in the Large Magellanic Cloud, and improved orbits for previously known systems. We compare orbital parameters of selected binaries containing O and WR type components. We also discuss the present status of knowledge for massive binary stars in the Magellanic Clouds and the problems encountered in their orbital studies such as stellar winds the ubiquitous tendency to be born in multiple systems.

Eclipsing binaries (EBs) have long been known for providing accurate stellar fundamental properties, such as masses and radii. These are obtained from the modeling of light and radial velocity curves and, with input data of good quality, uncertainties of a few percent in the components’ physical properties are now routinely achieved. Additionally, a number of other observables are determined for eclipsing binaries using techniques similar to those employed for single stars: Effective temperatures from multi-wavelength photometric analysis and chemical abundances from detailed spectroscopic modeling. Thus, EB systems provide a complete characterization of the physical properties of their components, with the added constraint that both stars should have identical age. Also worth mentioning is the ubiquity of EB systems among all kinds of stars. Main sequence stars, variable stars, giants, supergiants, and compact objects, to name a few, are found as members of EBs. This makes EBs excellent “laboratories” for stellar astrophysics. A number of studies have exploited this fact and carried out detailed analysis of galactic EB systems (e.g. Andersen et al. 1991; Clausen 1991; Claret & Giménez 1993; Schröder et al. 1997; Guinan et al. 2000). A few areas where EBs play a crucial role in astrophysics are stellar structure and evolution, tidal evolution, stellar atmospheres, binary evolution in interactive systems,…

Making use of detached eclipsing binaries with B-type components discovered by the OGLE and MACHO teams in the SMC and in the LMC, we give the value of the fractional radius above which circularization occurs. This critical radius is around 0.24 to 0.26, regardless of the mass, surface gravity or metallicity, and is consistent with that found by Giuricin et al. (1984) for galactic binaries. These empirical facts are shown to be consistent with Zahn’s (1975) theory of tidal dissipation. This work confirms and extends that of North & Zahn (2003), thanks to a sample of 448 binaries taken from the recent OGLE catalogue of 2580 eclipsing binaries in the LMC (Wyrzykowski et al. 2003) and a more homogeneous interpretation of the lightcurves of the 148 SMC binaries.

Large scale photometric surveys can deliver very large numbers of eclipsing binary stars. It is not presently possible to obtain radial velocity information for more than a small fraction of these. We have made some progress in the estimation of the statistical distributions of orbital elements (including semi-major axis and eccentricity) in the MACHO Project catalog of eclipsing binary stars. We see the well-known tendency to circularization in short period orbits and also detect late tidal circularization during the giant phase. The extension of these techniques to newer surveys will also be discussed.

Type la Supernovae have taken on a leading role in measuring extra-galactic distances providing key information about the Hubble Constant and providing evidence of an accelerating Universe. Unfortunately these conclusions are based on an empirical relationship between SN brightness and light curve shape rather than any deep physical understanding. A key ingredient to gaining physical understanding lies in understanding what becomes a SN la progenitor. Essentially all models require a binary system to push a white dwarf higher in mass. I will review the current attempts observational and theoretical to pin down the nature of these important astrophysical objects.

Core-collapse supernovae (SNe) come in a variety of subtypes. The SNe II-plateau (II-P) show a long plateau in their light curves, as well as classical P Cygni Balmer line profiles in their spectra. The SNe Il-linear (II-L) show a linear decline in their light curves after maximum, and the blue absorption trough in the Balmer lines is missing, leaving only broad emission profiles. The SNe II-narrow (IIn) show a narrow profile atop a broad base to the Balmer emission. The SNe lb exhibit no hydrogen lines, but prominent He lines instead. The SNe Ic also show no H, but also weak or no He, with their spectra dominated by intermediate-mass elements. The SNe lib start out appearing as SNe II(P), but in time evolve spectrally to resemble SNe lb, with a later resumption of broad Balmer profiles, as in the case of SN 1993J (see Filippenko 1997 for a review of SN types). The progression from SNe II-P, II-L, Iln, lib, lb, through Ic may be due to the degree of stripping of the H-rich stellar envelope. Binary star systems are likely the progenitors of many core-collapse SNe. In fact, we might expect ≳40-50% of all such SNe to arise from binaries (e.g., De Donder & Vanbeveren 1998).