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Owing to their similarities with giant exoplanets, brown dwarf companions of stars provide insights into the fundamental processes of planet formation and evolution. From their orbits, several brown dwarf companions are found to be more massive than theoretical predictions given their luminosities and the ages of their host stars 1 – 3 . Either the theory is incomplete or these objects are not single entities. For example, they could be two brown dwarfs each with a lower mass and intrinsic luminosity 1 , 4 . The most problematic example is Gliese 229 B (refs.  5 , 6 ), which is at least 2–6 times less luminous than model predictions given its dynamical mass of 71.4 ± 0.6 Jupiter masses ( M Jup ) (ref.  1 ). We observed Gliese 229 B with the GRAVITY interferometer and, separately, the CRIRES+ spectrograph at the Very Large Telescope. Both sets of observations independently resolve Gliese 229 B into two components, Gliese 229 Ba and Bb, settling the conflict between theory and observations. The two objects have a flux ratio of 0.47 ± 0.03 at a wavelength of 2 μm and masses of 38.1 ± 1.0 and 34.4 ± 1.5  M Jup , respectively. They orbit each other every 12.1 days with a semimajor axis of 0.042 astronomical units ( au ). The discovery of Gliese 229 BaBb, each only a few times more massive than the most massive planets, and separated by 16 times the Earth–moon distance, raises new questions about the formation and prevalence of tight binary brown dwarfs around stars. Analysis of the cool brown dwarf Gliese 229 B suggests that it is actually a close binary of two less massive brown dwarfs, explaining its low luminosity and settling the conflict between theoretical predictions and measurements.

Hierarchical triple systems comprise a close binary and a more distant component. They are important for testing theories of star formation and of stellar evolution in the presence of nearby companions. We obtained 218 days of Kepler photometry of HD 181068 (magnitude of 7.1), supplemented by ground-based spectroscopy and interferometry, which show it to be a hierarchical triple with two types of mutual eclipses. The primary is a red giant that is in a 45-day orbit with a pair of red dwarfs in a close 0.9-day orbit. The red giant shows evidence for tidally induced oscillations that are driven by the orbital motion of the close pair. HD 181068 is an ideal target for studies of dynamical evolution and testing tidal friction theories in hierarchical triple systems.

The study of the multiplicity of massive stars gives hints of their formation processes and their evolution path. Optical interferometry is mandatory to fulfill our knowledge of their multiplicity by probing the separation gap between 1 and 50 mas. We demonstrated the capability of the new interferometric instrument MIRC-X, located at the CHARA array, to study a large sample of more than 120 (H < 7.5) O-type stars. We observed 29 O-type star systems, including a couple of systems in average atmospheric conditions around a magnitude of H = 7.5. Out of these 29 systems, we detected 18 companions in 16 different systems, resulting in a multiplicity fraction f = 16 / 29 = 0.55, and a companion fraction of f = 18 / 29 = 0.62. We observed for the first time 11 of these detected companions. This study concludes that a large survey on more than 120 Northern O-type stars is possible with MIRC-X.

Tight relationships exist in the local Universe between the central stellar properties of galaxies and the mass of their supermassive black hole (SMBH) 1 – 3 . These suggest that galaxies and black holes co-evolve, with the main regulation mechanism being energetic feedback from accretion onto the black hole during its quasar phase 4 – 6 . A crucial question is how the relationship between black holes and galaxies evolves with time; a key epoch to examine this relationship is at the peaks of star formation and black hole growth 8–12 billion years ago (redshifts 1–3) 7 . Here we report a dynamical measurement of the mass of the black hole in a luminous quasar at a redshift of 2, with a look back in time of 11 billion years, by spatially resolving the broad-line region (BLR). We detect a 40-μas (0.31-pc) spatial offset between the red and blue photocentres of the Hα line that traces the velocity gradient of a rotating BLR. The flux and differential phase spectra are well reproduced by a thick, moderately inclined disk of gas clouds within the sphere of influence of a central black hole with a mass of 3.2 × 10 8  solar masses. Molecular gas data reveal a dynamical mass for the host galaxy of 6 × 10 11  solar masses, which indicates an undermassive black hole accreting at a super-Eddington rate. This suggests a host galaxy that grew faster than the SMBH, indicating a delay between galaxy and black hole formation for some systems. Using the GRAVITY+ instrument, dynamical measurement of the black hole mass in a quasar at a redshift of 2.3 (11 billion years ago) shows how the relationship between galaxies and black holes evolves with time.

Direct detection of thermal emission from nearby hot Jupiters has greatly advanced our knowledge of extrasolar planets in recent years. Since hot Jupiter systems can be regarded as analogs of high-contrast binaries, ground-based infrared long-baseline interferometers have the potential to resolve them and detect their thermal emission with precision closure phase—a method that is immune to the systematic errors induced by the Earth’s atmosphere. In this work, we present closure-phase studies toward direct detection of nearby hot Jupiters using the CHARA interferometer array outfitted with the MIRC instrument. We carry out closure-phase simulations and conduct a large number of observations for the best candidate υ And. Our experiments suggest that the method is feasible with highly stable and precise closure phases. However, we also find much larger systematic errors than expected in the observations, most likely caused by dispersion across different wavelengths. We find that using higher spectral resolution modes (e.g., R = 150 R = 150 ) can significantly reduce the systematics. By combining all calibrators in an observing run together, we are able to roughly recalibrate the lower spectral resolution data, allowing us to obtain upper limits of the star-planet contrast ratios of υ And b across the H H band. The data also allow us to get a refined stellar radius of1.625 ± 0.011 R ⊙ 1.625 ± 0.011     R ⊙ . Our best upper limit corresponds to a contrast ratio of2.1 × 103 : 1 2.1 × 10 3 : 1 with 90% confidence level at 1.52 μm, suggesting that we are starting to have the capability of constraining atmospheric models of hot Jupiters with interferometry. With recent and upcoming improvements of CHARA/MIRC, the prospect of detecting emission from hot Jupiters with closure phases is promising.

The mass of a Cepheid is a fundamental parameter for studying the pulsation and evolution of intermediate-mass stars. But determining this variable has been a long-standing problem for decades. Detecting the companions (by spectroscopy or imaging) is a difficult task because of the brightness of the Cepheids and the close orbit of the components. So most of the Cepheid masses are derived using stellar evolution or pulsation modeling, but they differ by 10-20 %. Measurements of dynamical masses offer the unique opportunity to make progress in resolving this mass discrepancy. The first problem in studying binary Cepheids is the high contrast between the components for wavelengths longer than 0.5 μm, which make them single-line spectroscopic binaries. In addition, the close orbit of the companions (<40 mas) prevents us from spatially resolving the systems with a single-dish 8m-class telescope. A technique able to reach high spatial resolution and high-dynamic range is long-baseline interferometry. We have started a long-term program that aims at detecting, monitoring and characterizing physical parameters of the Cepheid companions. The GAIA parallaxes will enable us to combine interferometry with single-line velocities to provide unique dynamical mass measurements of Cepheids.

We give an update on our long-term program of Galactic Cepheids started in 2012, whose goal is to measure the visual orbits of Cepheid companions. Using the VLTI/PIONIER and CHARA/MIRC instruments, we have now detected several companions, and we already have a good orbital coverage for several of them. By combining interferometry and radial velocities, we can now derive all the orbital elements of the systems, and we will be soon able to estimate the Cepheid masses.

Cepheids are one of the most famous standard candles used to calibrate the Galactic distance scale. However, it is fundamental to develop and test independent tools to measure their distances, in order to reach a better calibration of their period-luminosity (P-L) relationship. We present here the first results obtained with the Integrated Parallax of Pulsation (IPoP) method, an extension of the classical Baade-Wesselink method that derives the distance by making a global modelisation of all the available data. With this method we aim to reach a 2% accuracy on distance measurements. Cepheid masses are also an essential key for our comprehension of those objects. We briefly present an original approach to derive observational constraint on Cepheid masses. Unfortunately, it does not lead to promising results.

Optical interferometry is the only technique giving access to milli-arcsecond (mas) resolution at infrared wavelengths. For Cepheids, this is a powerful and unique tool to detect the orbiting companions and the circumstellar envelopes (CSE). CSEs are interesting because they might be used to trace the Cepheid evolution history, and more particularly they could impact the distance scale. Cepheids belonging to binary systems offer an unique opportunity to make progress in resolving the Cepheid mass discrepancy. The combination of spectroscopic and interferometric measurements will allow us to derive the orbital elements, distances, and dynamical masses. Here we focus on recent results using 2- to 6-telescopes beam combiners for the Cepheids X Sgr, T Mon and V1334 Cyg.

Luminous Blue Variables (LBVs) are a brief phase in the evolution of massive stars, but a very important one. The giant eruptions remain enigmatic, but the discovery of the flagship LBV η Car to be a five-year highly eccentric binary put focus on possible binarity induced mechanism for these outbursts, and prompted binarity searches among LBVs. While several wide LBV binaries were identified, HR Car is the first system found to be similar to η Car, i.e., relatively close & eccentric.