It is appropriate that this meeting about double and multiple stars be dedicated to Arcadio Poveda, who has derived so many results about such systems. This dedication mentions some of those results, as well as some other details of his research and accomplishments.

The statistics of stellar systems of multiplicity three and higher is reviewed. They are frequent, 0.15−0.25 of all stellar systems. Some 700 multiples are expected among the 3383 stars of spectral type F, G, and K within 50 pc, while only 76 of them are actually known. Many (if not all) close binaries have distant tertiary components, indicating that angular momentum exchange within multiple systems was probably critical in forming short-period binaries. The ratio of outer to inner periods in the best-studied nearby multiples and in low-mass pre-main sequence multiples does not exceed 104 at the formation epoch; larger ratios are produced by subsequent orbital evolution. All multiples with well-defined orbits are dynamically stable, the eccentricities of outer orbits obey the empirical stability limit Pout(1 – eout)3/Pin > 5 that is more strict than current theoretical limits. Relative orientation of orbits in triple stars shows some degree of alignment, especially in weakly-hierarchical systems. The statistics support the idea that most multiple stars originated from dynamical interactions in small clusters.

We examine the correlation of projected rotational velocities in binary systems. It is an extension of previous work (Steinitz and Pyper, 1970; Levato, 1974). An enlarged data basis and new tests enable us to conclude that there is indeed correlation between the projected rotational velocities of components of binaries. In fact we suggest that spins are already correlated.

Two Coravel radial velocity surveys dedicated to F7-K field dwarfs and to open clusters are merged in order to investigate the statistical properties of binaries with periods up to 10 years. Thanks to the accurate trigonometric parallaxes provided by Hipparcos, an unbiased sample of spectroscopic binaries (SB) is selected. After correction for the uncertainties of the measurements, the following results are obtained: 1. The distribution of mass ratios exhibits a peak for equal-mass binaries (twins), which is higher for short-period binaries than for long-period binaries. 2. Apart from the twins, the distribution of mass ratios exhibits a broad peak from 0.2 to 0.6. 3. The orbital eccentricities of twins are slightly smaller than those of other binaries. 4. An excess of SB is observed with periods shorter than about 50 days in comparison with the Duquennoy and Mayor log-normal distribution of periods. These features suggest that close binary stars are generated by two different processes. A possible difference could come from the accretion onto the binary, for instance from a common envelope or from a circumbinary disk. Alternatively, twins could come from dynamic evolution of multiple systems. It is not clear whether the formation models are already sufficiently elaborated to reproduce our statistics.

We have undertaken a new investigation of the statistical properties of main-sequence binaries, which is a revision and extension of the Duquennoy & Mayor survey of solar-type stars. The analysis has been divided into two parts: the spectroscopic binaries with periods shorter than 10 years, and the long-period systems including visual binaries and common proper motion pairs. In this contribution we present preliminary results regarding the intrinsic mass ratio and period distributions of visual binaries. Our results are strongly limited by small-number statistics, but when combined with the ones found for the spectroscopic binaries, the following results are obtained: (i) the excess of equal-mass binaries, if still present, is less important for long-period systems than for the binaries with P < 50 days; (ii) the period distribution is inconsistent with a flat distribution.

Population II stars are old, metal-poor, Galactic halo stars with high proper motion. We have carried out a visual binary survey of 164 halo stars in the solar neighborhood (median distance 100 pc), using infrared speckle interferometry, adaptive optics, and wide field direct imaging. The sample is based on the lists of Population II stars of Carney et al. (1994) and Norris (1986), with reliable distances from HIPPARCOS measurements.

At face value, we found 33 binaries, 6 triples, and 1 quadruple system. When we limit ourselves to K-band flux ratios larger than 0.1 (to avoid background contamination), the numbers drop to 9 binaries and 1 triple, corresponding to a binary frequency of 6–7% above our angular resolution limit of about 0.1 arcsec. If we count all systems with K-band flux ratios greater than 0.01, we obtain 15 more binaries and 3 more triples, corresponding to a binary frequency for projected separations in excess of 10 AU of around 20 %. This is to be compared with the frequency of spectroscopic binaries (up to a period of 3000 days) of Population II stars of about 15 % (Latham et al. 2002). We also determined a semi-major axis distribution for our visual Population II binary and triple systems, which appears to be remarkably different from that of Population I stars. Second epoch-observations must help confirm the reality of our results.

Two previous studies of the secondary mass function in spectroscopic binaries by Abt & Levy (1976) and by Duquennoy & Mayor (1991) are shown to be in good agreement if they are both plotted with the same abscissa scale. A new study of 271 main-sequence stars later than F6 V made with a radial-velocity accuracy of ±0.10 km s-1 yielded 10 new sets of orbital elements in addition to the 59 published ones. The resulting secondary mass function is nearly flat and shows that 2.2±1.5% of the primaries have low-mass (0.01–0.10 M⊙) companions. In contrast, the secondary mass function for visual binaries with separations >500 AU fits a van Rhijn function, as was shown previously by Abt and Levy.

We observed the hierarchical triple star Eta Virginis with the Navy Prototype Optical Interferometer and present the first images resolving all three components. The stars in the close pair have a separation between 4 and 9 milliarcseconds. The orbital motions of the stars were followed during the winter and spring of 2002. Preliminary, astrometrically determined orbits of the two components in the close pair by reference to the tertiary were derived, thus enabling the estimation of the mass ratio (1.27) of the components in the close pair. We also determined the relative orbital inclination to be 31 degrees. We describe the model-fitting process to interferometrie and spectroscopic data, and discuss some astrophysical component parameters.

High-dispersion spectroscopic observations of 77 Cygni, V815 Herculis, and HD 140122 have resulted in the detection of at least one additional component in each system. The newly identified component of 77 Cyg has a period of 35.48 days. V815 Her is a triple system with periods of 1.810 and 2300 days or 6.3 yr. The two visual binary components of HD 140122 are short-period spectroscopic binaries with periods of 10.879 and 15.770 days.

For the purpose of understanding how the distribution of major semiaxes of double and multiple stars has evolved from the earliest ages up to the ages of the oldest systems, we have studied a number of different groups of such systems. We review and expand our previous work on the following groups of stars: (a) The wide binaries in the IDS; (b) Luyten’s common proper motion pairs (the LDS catalogue), (c) Our catalogue of nearby wide binaries divided into two groups: the youngest and the oldest; (d) The common proper motion pairs in the Orion Nebula cluster; (e) Our sample of high velocity, low metallicity binaries that represents a population of very old systems. From the very young binaries (Orion Nebula Cluster) to the oldest (thick disk and halo), all groups show a distribution of major semiaxes following Oepik, i.e., f(a) da ∝ da/a (where a is the major semiaxis), valid over an interval of a between 60 AU and am(t), am(t) being a maximum semiaxis which depends on the age of the binaries as well as on the number density and velocities of the massive objects they encounter as they travel in the Galaxy. Strong gravitational encounters in recently formed multiple systems were found to produce an Oepik distribution, which at small separations (a ≤ 40 − 60 AU) gets truncated by circumstellar disks and close binaries, and at large separations gets depleted by encounters with massive objects.

We compile most recent multiplicity fractions and binary separation distributions for different primary masses, including very low-mass and brown dwarf primaries, and compare them with dynamical decay models of small-N clusters. The model predictions are based on detailed numerical calculations of the internal cluster dynamics, as well as on Monte-Carlo methods. Both observations and models reflect the same trends: (1) The multiplicity fraction is an increasing function of the primary mass. (2) The mean binary separations are increasing with the system mass in the sense that very low-mass binaries have average separations around ≈ 4AU, while the binary separation distribution for solar-type primaries peaks at ≈ 40AU. M-type binary systems apparently preferentially populate intermediate separations. Similar specific energy at the time of cluster formation for all cluster masses can possibly explain this trend.

Improved orbital elements for ϕ Cygni have been determined from 47 new high-dispersion spectroscopic observations. The period and eccentricity are 434.14 days and 0.559, respectively. Minimum masses plus the inclination of the visual orbit result in masses of 2.16 and 2.06 M⊙ for the two giants. The small magnitude difference of 0.25 mag and a mass ratio of 1.044 indicate that both giants are most likely in the helium-burning phase of their evolution.

We derive the period (P), primary mass (m1) and mass ratio (q) distributions of the local population of field binaries by studying a volume-limited sample of 371 spectroscopic binaries (SBs) in the solar neighbourhood d ≤ 100 pc and Mv ≤ 4. The sample was collated using the Batten catalogue, data of R.F. Griffin and the Hipparcos catalogue. The SB2s are used to calibrate a Monte-Carlo approach to the q distribution of SB1s, giving a total q distribution confirming a peak at q ≈ 1. Completenesses and parameter-specific biases are also assessed. A substantial number of systems with intermediate to long periods are found which may have significant consequences for the mass-distribution of WDs.

Some progress has been made towards determining the long-period outer orbits of certain spectroscopic multiple stars.

A program of follow-up observations of HIPPARCOS Double Stars has been started using the WIYN 3.5-m Telescope at Kitt Peak, Arizona. Observations are made with the recently-completed RYTSI speckle camera, which is capable of determining reliable magnitude differences at various wavelengths down to the diffraction limit of the telescope. Thus in addition to precise relative astrometry, component magnitudes and colors can be obtained in a survey mode for all systems observed. Preliminary results and system capabilities are described. The benefits of a sustained program of observations of the HIPPARCOS Double Stars include populating the mass-luminosity relation with many more points (once the orbital status of these stars is known and orbital parameters are determined in sufficient detail for gravitationally bound pairs), robust tests of stellar evolution calculations in cases where at least one component has evolved off the main sequence, and a more complete picture of the nearby field binary population.

We present the first eccentricity-period diagram for binaries among nearby field M giants, where an observed frequency of at least 8% of spectroscopic binaries is found.

New uvby photoelectric photometry has been acquired for the triple star DG Leo at two different observatories equipped with analogous instrumentation. A preliminary period analysis indicates the presence of at least 3 close δ Scuti frequencies (10-12 c/d, 3-6 mmag) and a slow variation. This slow variation fits quite well with half the orbital period of the spectroscopic binary; the noise level in the amplitude spectrum is only 3–4 mmag (after prewhitening).