The primary result of lunar occultation studies of multiple star systems is the discovery of new systems. Its use as a method of multiplicity discovery has been productive, although incidental to timing observations made by amateurs and professionals for 360 years. The capabilities and limitations as a method of discovering new double stars is discussed.

By improving on the angular resolution possible with conventional telescopes, speckle interferometry has been adding to the list of known masses, resolving binaries with separations of 30 mas. Interferometry with separate apertures offers the possibility of going to much higher resolution.

The Mark III Optical Interferometer on Mt. Wilson has been used for astrometry, for stellar diameter measurements, and for binary observations since routine operations began in late 1988. The fringe visibilty calibration uncertainty is ~1% for m < 4m at λ800 nm with good seeing, so that both binary components can be detected when the magnitude difference is 3.5m to 4m.

From Mark III data, masses of eight stars have been determined. The most precise determination is for the components of the ϕ Cygni system: 2.545 ± 0.085 and 2.445 ± 0.081 M⊙. To improve on this measurement, more precise spectroscopic data are needed, as is the case for most of the systems for which we have orbits or preliminary orbits.

The NRL/USNO Optical Interferometer Project is currently designing two more capable instruments: a four–element system for astrometric observations, and a six–element system for imaging. The ultimate 430-m maximum baseline and 35-cm apertures of the imaging interferometer will allow observations to 8th to 10th mag and resolution of binary components separated by as little as ≈ 200 µas.

Long–baseline optical interferometry has revolutionised binary star astronomy. This paper reviews some recent binary star work done using the Mark III Stellar Interferometer. For the first time, many spectroscopic binaries, eclipsing binaries, triple systems, and binaries with X-ray sources have had their visual orbit, diameter of the primary component, and color index determined. Also, accurate stellar masses, distances, radii, and effective temperatures have been derived. A preliminary distance to the Hyades cluster has also been determined. In addition, preliminary observations provide evidence of a third body in the binary star system ζ Her, which may resolve an 85-year old discrepancy in astronomy.

The “Grand Interféromètre à deux Télescopes” (GI2T) is presently the largest operational interferometer operating at visible wavelengths (Labeyrie et al. 1986, Mourard et al. 1991). With two telescopes of 1.5-m aperture and baselines reaching 65 m in the North–South direction, it gives stellar spectra containing interference fringes, which provide high angular resolution measurements in the range of one milliarcsecond.

The GI2T optical combination follows the Michelson principle of output pupil remapping to preserve a constant fringe spacing on the detector for stable and economical spatial sampling. The stellar light is dispersed in order to study the fringes parameters simultaneously with the spectral information. The direction of dispersion is perpendicular to that of the recombined pupils. Due to atmospheric turbulence the instantaneous images contain about 100 speckles. To freeze the atmospheric turbulence a temporal resolution of at least 10 milliseconds is needed.

The Sydney University Stellar Interferometer (SUSI) is currently undergoing commissioning and will soon commence its astronomical program in which observations of double stars will form a major component. With its 640-m long North–South array of input siderostats, the new instrument will have unprecedented angular resolution.

Interferometric arrays possessing sub-milliarcsecond resolution are either about to be fully scientifically productive, as in the case of the Sydney University Stellar Interferometer, or are under various stages of planning and development. The 1990’s will thus witness a hundred–fold gain in resolution over speckle interferometry at the largest telescopes and 5,000 times the resolution of classical direct imaging. Where speckle interferometry can now resolve binary stars with periods of 1 to 2 years, interferometric arrays with baselines of hundreds of meters will resolve binaries with periods of a few hours. Arrays will resolve the majority of the known spectroscopic binaries, providing a substantial increase in the quantity and quality of stellar mass determinations. Surveys for new binaries among the field stars and other restricted samples will be accomplished with unprecedented completeness. The remarkable enhancement in resolution we are about to witness from facilities like SUSI and our own proposed CHARA Array will quite literally revolutionize the field of double and multiple star research.

Reduction of speckle data obtained for binary stars is typically carried out using power spectrum or, equivalently, autocorrelation methods. An especially powerful algorithm from which accuate differential astronomy can be obtained is the vector autocorrelation technique. While such methods are highly suited to extracting astrometric information from very large volumes of speckle data in near real–time, they inherently introduce a 180° ambiguity in position angle measurement. We briefly summarize results with a new algorithm which maintains most of the simplicity of vector autocorrelation while removing the quadrant ambiguity.

Image reconstruction techniques have rapidly matured over the last decade, and increasing reference to full–aperture and masked aperture results appear in the literature. Although anomalies in morphology are understood, most work uses nonlinear iterative refinements of questionable photometric robustness. Recent development of linear least–squares phase reconstruction techniques from the bispectrum (Glindemann et al. 1991) offers hope for a robust technique using a computational tour de force. However, the field cannot be considered mature until routine photometry of a large number of speckle–resolved objects is performed and demonstrated to be repeatable. We review our work and our expectations for use of both computationally intensive and simple techniques in our routine speckle reductions.

We present a preliminary report on radial–velocity and infrared interferometric observations, with emphasis on the newly resolved nearby sources Gl 609.2 and Gl 804. We briefly discuss their low–mass companions, their luminosities, and their individual masses inferred from the combined solution of their spectroscopic and visual orbits.

We present a preliminary report on infrared interferometric observations, with emphasis on the resolved nearby sources Gl 563.4, Gl 570B and Gl 773.3. We briefly discuss their individual masses inferred from the combination of spectroscopic and visual data.

We first briefly recall the geometry of the occultation of a double star by the Moon’s edge. Then we give a short description of the principle of the formation of the diffraction pattern. We present the results for three double stars and compare them with those obtained by other methods.

We have made two sets of speckle observations of binary stars with the Multi-Anode Microchannel Array (MAMA) detector. Our observing system is a true photon counting imaging device which records the arrival time of every detected photon. We present speckle autocorrelation analyses of five binary systems, two observed with the 3.6-m telescope at the European Southern Observatory and three observed with the 40-in reflector at Lick Observatory. These five systems represent a wide variation in separation and magnitude difference that indicate that the MAMA detector is capable of recording high quality speckle data at extremely low count rates and can recover image features very near the diffraction limit of the telescope. In one case, only 10 photons per frame were recorded for the dim companion of the system, and in another case, a separation of 0″.157±0″.031 was derived for a system observed with the 40-in telescope where the diffraction limit is about 0″.125. Future prospects for this system are discussed.

Using the variable baseline of the Mark III Optical Interferometer (Shao et al. 1988) with lengths ranging from 3 to 31.5 m, we measured the fringe visibility of stars in three 25 nm bandpass filters, at wavelengths between 800 nm and 450 nm. This setup gives fringe spacings ranging from 55 mas to 3 mas.

255 stars brighter than fifth magnitude and north of –10° declination were selected from the Batten et al. (1989) catalog of spectroscopic binaries. We compiled values for the orbital axes from the McAlister & Hartkopf (1988) catalog. If unknown (in the majority of cases), they were estimated from the masses predicted from the spectral types, the periods (if known), and the distances to the binaries. Priority was given to the binaries with orbital axes between 3 mas and 40 mas. Observational procedures and the calibration of the data are described by Armstrong et al. (1992). Statistical and calibration uncertainties are in the range of 1% (red channel) to 10% (blue channel).

Our speckle observations were started in 1987 using a 212-cm telescope at San Pedro Martir Observatory in Mexico. A D-IMOSS camera was used until 1990 and an I-CCD camera was introduced in 1991. The old camera has only 0″.20 angular resolution because of an aging effect in its intensifier, while the new camera has 0″.06 angular resolution, which is nearly equal to the diffraction limit of the telescope. 128 visual binary stars and 755 spectroscopic binary stars have been observed. The numbers of stars and observations each year are shown in Table 1 and all the observed data are given in a series of papers by Isobe et al. (1990a, 1990b, 1992a) and Miura et al. (1992).

Recent high–angular resolution speckle interferometry measurements of the positions of Mira’s companion are presented in context of the previous visual observations. The analysis of the separation measurements since 1923 show evidence for possible third body in the system.

Due to its proximity to the Sun, the Hyades serves the critical role of luminosity calibration of all cluster main sequences, and hence is one of the lower rungs in the cosmic distance ladder. We attempt here to use the enhanced capabilities of speckle interferometry, in comparison with classical techniques, to add to the list of binary and multiple stars in the Hyades. New systems will not only eventually help to improve our knowledge of the cluster distance, but they will also help further our understanding of the formation and evolution of binary and multiple stars in the cluster environment.

The F7V star HR 1071 has attracted little individual attention in the literature. Eggen (1972, 1973) included HR 1071 among some 250 metal–deficient stars, of which 20 are halo population objects, in a photometric calibration of their absolute magnitudes. HR 1071 stands out in Eggen’s sample as a disk population star of extreme space velocity compared with stars of similar moderate metal deficiency, having (U, V, W) = (−60, −39, +8) km s−1. Kinematically, HR 1071 would be considered an old disk population star although this classification would be strengthened if the metallicity were lower. More recently, Balachandran (1990, hereafter BAL) took Eggen’s (1973) sample as a basis for studying the possible effects of rotational breaking on Li depletion in old–disk population F stars. HR 1071 possesses the fifth lowest Li abundance (the Li λ6707 Å equivalent width was less than 1 mÅ) and the third lowest observed rotational velocity of 5 km s−1 in BAL’s sample.

HR 6469 consists of an evolved G5 star and a close pair of fainter stars, probably both on the main sequence. The period of the close pair is just over two days, and shallow eclipses have been detected (Boyd et al. 1985), although no analysis of the light curve has been published. The wide system has a period of about 5.5 years, and has been resolved by speckle interferometry (McAlister et al. 1983). The G5 star and the primary of the close pair are detectable in the spectrum, with the latter showing modest rotational broadening. Spectral types and rotational velocities for the evolved star and the brighter component of the close pair have been published by Strassmeier & Fekel (1990).

Lunar occultation data often are reduced by means of a multiparameter least-squares fitting routine. For binary stars the minimum number of free parameters is five: the time of geometric occultation for each star, the intensity of each component, and the background intensity. From these values the magnitude difference between components can be calculated as well as the angular separation of the binary in the direction perpendicular to the lunar limb (i.e. the lunar position angle). If the observed rate of motion of the lunar limb can be determined as an additional free parameter, then the difference between observed and predicted values can be interpreted in terms of a local lunar slope. Therefore, in principal the observed rate of motion can be used to correct the vector separation — both angular separation and direction — of the binary. In this paper four occultation binaries observed from KPNO are examined to determine whether binary-star astrometry can be improved using these corrections.

The existing and new observing techniques for combined speckle–spectroscopic studies of binary stars are briefly reviewed. The late–type dwarf stars are particularly rewarding since simultaneous spatial and spectroscopic resolution is possible for a large number of systems. A list of new candidates for the interferometric resolution is given. These studies can make an important contribution to the data on stellar masses and also provide some insight into the binary formation mechanisms when applied to multiple systems.