In this communication the multiple systems Hu 1399 and Cor 197 have been studied. A deviation of the residuals has been established. The relative rectilinear motion of the components AC of the system Hussey 1399 can be used to obtain information about the invisible companion.

Observations made during the 70 last years of the visual binary star Aitken 14 reveal systematic deviations in its motion, suggesting the existence of an astrometric companion.

Selected topics of double star photometry are discussed together with current observational programs and new observing techniques. A revised catalog of visual binaries with one or more variable components is described.

Analysis of the radial velocity variation of θ Virginis A has revealed that, superposed on an orbital variation with a period of 17 years, there is present an ultrashort periodic variation of 0.15-day. This secondary cycle is attributed to pulsation. The question of what type of pulsating star θ Virginis A may represent is discussed.

The orbital histories of the systems Sirius AB, Procyon AB, 40 Eri BC and Stein 2051 AB are examined. It is assumed that the changes in the orbital elements are a direct result of mass loss from the white dwarf precursor component. This loss is considered to be isotropic and no exchange between the components is allowed. These histories, coupled with observed atmospheric abundances, theoretical ages and cooling times lead to the conclusion that stars as massive as 10M⊙ can be white dwarf progenitors. Indeed, in the case of Sirius B, there is evidence that the progenitor mass was not less than 8M⊙ and that the star did not lose mass in an explosive manner.

A catalogue of more than 900 multiple systems of trapezium type has recently been completed. In constructing the catalogue an attempt was made to exclude optical systems. We study the galactic distribution of trapezia of different spectral types, and we compare it with that of ordinary multiple systems and of visual binaries from the IDS. An estimate is made of the expected number of pseudo-trapezia among each spectral type. This study shows that most of our trapezia are true trapezia and should therefore be young. This conclusion holds for trapezia of all spectral types.

From dynamical considerations, a significant fraction of multiple systems of trapezium type is expected to be quite young. A search for positional coincidences between trapezia from various catalogues and infrared sources at 2.2 microns was undertaken. It was found that between 12% and 20% of the trapezia can be associated with infrared sources. Various statistical tests indicate that these coincidences are not likely to be accidental. The nature of the infrared sources associated with trapezia is briefly discussed.

We present UBVRI photometric data for 50 stars forming 14 systems of the trapezium type. Color-magnitude and color-color diagrams indicate the presence of ultraviolet excess and anomalous behavior in the infrared part of the spectrum. This is consistent with the dynamical ages for stars in trapezia.

Four ways of studying the dynamics of triple systems are outlined, namely the analytical, the qualitative, the numerical, and the observational approach. A discussion of the stability of classical (revolutional, hierarchical) triples summarizes the results of analytical developments, based on the application of von Zeipel’s method to the general three-body problem, and of numerical experimentation. Finally, a summary is given of recent investigations concerning the dynamics of triples other than the revolutional systems.

A classification of possible motions of triple systems is presented emphasizing the transient phenomena occurring in addition to the final (asymptotic) outcome and clarifying the discrepancies between the astronomical and mathematical formulations. A conjectured possible instability is described and it is shown that systems with negative total energy and low angular momentum may lead to instability and to the formation of binaries. The ejected or escaping star may have high velocity if the triple close approach preceding the escape is sufficiently close. The computational results of several systematic series of such escapes are applied to various stellar configurations.

The present status of the fundamental problem of partitioning the phase-space into stable and unstable regions is reviewed and a recently developed technique, applicable to stellar dynamics is described. Recently discovered families of periodic orbits and previously established classical configurations are shown to weaken the general instability conjecture.

The possible existence of triple systems in states of dissolution offer intriguing observational challenges regarding the discovery of these projected temporary trapezium type systems.

We investigate by numerical exploration the extended regions of stability for retrograde satellites when the mass of the less massive body increases up to μ = 0.5. A preliminary study is made of the characteristic and stability properties of the families of periodic orbits; then the general case of non-periodic orbits is considered. Those numerical explorations show that for stable large non-periodic orbits, the motion can be decomposed into a fast “reference motion” and slow libration; we study theoretically this libration in Hill’s case for which the “reference motion” is elliptic.

We give a complete description of a one-parameter family of periodic orbits in the planar problem of three bodies with equal masses. This family begins with a rectilinear orbit, computed by Schubart in 1956. It ends in retrograde revolution, i.e., a hierarchy of two binaries rotating in opposite directions. The first-order stability of the orbits in the plane is also computed. Orbits of the retrograde revolution type are stable; more unexpectedly, orbits of the “interplay” type at the other end of the family are also stable. This indicates the possible existence of triple stars with a motion entirely different from the usual hierarchical arrangement.

We consider a rectilinear periodic solution in which the central body collides alternately with each of the two other bodies. This solution is found to exist for all values of the three masses. Its stability with respect to three-dimensional perturbations is computed. Domains of stability and instability are delimited in a triangular mass diagram. Large domains of stability are found. This reinforces the conclusion that triple stars may have an “interplay” type of motion.

Numerical results are presented for calculations of stellar three-body systems. Among the triple systems with similar initial separations between each of the stars, the equal-mass systems are most stable with typical half-lives of about 40 crossing times. In the other extreme, systems with a very small total angular momentum or a large mass difference have half-lives of only about 10 crossing times. The escaping star is usually the lightest one, while the escape probability for the heaviest star approaches zero when its mass exceeds the combined mass of the other two stars by more than a factor of about 5. The final binary becomes very eccentric (ē ≳ 0.8) when the three stars are restricted to a plane and do not have a very high total angular momentum, or when the total angular momentum is very small.

Previous treatments by Ambartsumian and by Cruz-González are combined into a Fokker-Planck equation that describes the time evolution of the distribution of orbit sizes of a set of wide binaries, as encounters with field stars statistically change the orbits. A solution can be found that predicts the distribution of separations greater than about 3000 AU.

The concept of the “lifetime” of a population of binaries is applicable only to those of low binding energy, i.e. wide pairs. Of several possible methods of estimating a lifetime, that based on the average change in binding energy due to non-disruptive encounters appears to be the best.

In this paper we have discussed the origin of planetary systems on one hand and binary and multiple stars on the other. First we show that phenomenological differences between these two kinds of celestial objects are due to their genetic difference. The basic point is that formation of a planetary system around a star has to be a minor event in the life history of the star while formation of a binary or multiple system has to be an event that is important equally to all components of the system. Thus the planetary system evolves from a rotating disk of gaseous and dust particles that comes into being after the star has already been there. It is therefore reasonable to suggest that the rotating disk results from transfer of angular momentum from the central star to the surrounding medium which is likely a residue left over in the process of formation of the central star.

Binary and multiple systems cannot be formed in this way because they do not show the characteristics of having come out of a rotating disk. The dominant mechanism of their formation is that they were formed naturally as they are, each from perhaps a single condensation in the interstellar medium. However such a single mechanism of formation cannot satisfactorily explain the observed spread of binaries in mean separations between two components (or equivalently orbital periods). But the disagreement may be removed by including a small number of binaries formed by other processes and by considering the change of orbital elements of binaries after their formation. Trapezia were likely formed also by more than one mechanism.

That several stars could be formed, from a single condensation requires the” existence oí pre-stellar nuclei which are briefly: discussed at the end of the paper.

The classical theories of double star formation are briefly reviewed in relation to the origin of multiple stars. Modern observations in the infrared are discussed and their relevance as clues to the process of multiple star formation is stressed. These observations, together with dynamical considerations and data on the statistics of trapezia and hierarchical multiple systems, support the view that multiple stars, in general, are born as trapezia, that is, as independent condensations.

The region S106 = G 76.4 — 0.6 has been photographed in the 8000Å region with the Carnegie image converter attached to the 0.6m telescope at the Mees Observatory. This was done in connection with a larger program designed to assemble radio, infrared, and optical data on compact HII regions.

S106 has angular dimensions of 1’ by 3’ and has relatively high surface brightness. Its kinematic distance based on H109a observations (Reinfenstein, et al. 1970) is 2.3 kpc ± 3.0.

Several bright maxima are observed within the nebula at wavelengths of 12.6/x and 2.7 GHz. These are believed to be the result of thermal radiation from hot dust and free-free emission from ionized gas, respectively. At the nominal distance their linear sizes are of the order of 0.2 pc and they may be similar to the “cocoon” type objects discussed by various authors.

This review is mainly devoted to a discussion of binary formation and evolution in stellar systems, as described by N-body techniques. The simplest formation mechanisms consist of ejection from bound triple systems and capture arising from hyperbolic three-body encounters. However, the large number of astrophysically close binaries cannot be accounted for in this way unless most stars are formed in compact groups. Numerical calculations show that the evolution of clusters containing several hundred members is invariably dominated by one central binary which absorbs a large fraction of the total energy. It is suggested that a visual binary in the core of the Hyades cluster may have been associated with this process.