Young M-type binaries are particularly useful for precise isochronal dating by taking advantage of their extended pre-main sequence evolution. Orbital monitoring of these low-mass objects becomes essential in constraining their fundamental properties, as dynamical masses can be extracted from their Keplerian motion. Here, we present the combined efforts of the AstraLux Large Multiplicity Survey, together with a filler sub-programme from the SpHere INfrared Exoplanet (SHINE) project and previously unpublished data from the FastCam lucky imaging camera at the Nordical Optical Telescope (NOT) and the NaCo instrument at the Very Large Telescope (VLT). Building on previous work, we use archival and new astrometric data to constrain orbital parameters for 20 M-type binaries. We identify that eight of the binaries have strong Bayesian probabilities and belong to known young moving groups (YMGs). We provide a first attempt at constraining orbital parameters for 14 of the binaries in our sample, with the remaining six having previously fitted orbits for which we provide additional astrometric data and updated Gaia parallaxes. The substantial orbital information built up here for four of the binaries allows for direct comparison between individual dynamical masses and theoretical masses from stellar evolutionary model isochrones, with an additional three binary systems with tentative individual dynamical mass estimates likely to be improved in the near future. We attained an overall agreement between the dynamical masses and the theoretical masses from the isochrones based on the assumed YMG age of the respective binary pair. The two systems with the best orbital constrains for which we obtained individual dynamical masses, J0728 and J2317, display higher dynamical masses than predicted by evolutionary models.

Stars are involved in most research fields of astronomy, ranging from studies of faraway galaxies, exploding supernovae, to more nearby exoplanets and even our own Sun. As such, it is paramount that our physical interpretation of stars is accurate. By observing stars at different epochs, we can fashion evolutionary models to predict important events that occur at different phases during their life-cycle. Thus, exemplary stars where properties including mass, age and luminosity can be observed become increasingly valuable as benchmarks for calibrating said models with. Sometimes, all of these essential properties can be measured for a single system. For instance, for a binary star which circles a common centre of mass we can from its orbital motion calculate the dynamical mass of the system. If the stellar system also has a well-determined age we may use it as a benchmark for our models, and hence refer to it as an emblematic binary system.

In this thesis we are searching for exactly these emblematic binaries, both among lowmass stars and substellar brown dwarfs. We also show how to measure the different characteristics that make the systems into exemplary touchstones. We provide an overview over the different types of stellar binaries, how mass and age estimates are performed, as well as discuss the implications multiplicity has for the formation and evolution of stars and brown dwarfs. In Paper I we present the results from an orbital fit we constrained for a low-mass binary with a known age, making into a valuable and relatively rare benchmark. We also show in Paper II how long baseline astrometry can be exploited in order to place better constraints for orbital fits and dynamical masses for low-mass companions to stars by measuring the perturbation in proper motion over time. The dynamical masses are sequentially tested against evolutionary models, which at these low masses display several discrepancies compared to the observables, and are thus questioned. We explore more uncharted mass-regimes in Paper III, where we employ laser guide star assisted adaptive optics to search for multiplicity among faint substellar objects in young moving groups, detecting 3 new young brown dwarf binary systems. These new binaries will prove to be highly valuable systems for future research of brown dwarfs, and will be able to be studied further with for instance the Extremely Large Telescope or James Webb Space Telescope, which also makes them into prominent benchmarks for substellar evolutionary models. Furthermore, age estimation typically dominates the error budget for low-mass stars and brown dwarfs, requiring several different approaches for a robust assessment. In Paper IV we test and compare different techniques for age determination of 7 low-mass binary stars. These binaries have had their orbital motion monitored for a longer time, and will soon be constrained well enough that dynamical masses may be procured. As such, these low-mass binaries will extend the so far scarce number of exemplary systems where both mass, luminosity and age can be determined, to later be used to calibrate theoretical evolutionary models.

We present the results from an integral field spectroscopy study of seven close visual binary pairs of young M-dwarf multiple systems. The target systems are part of the astrometric monitoring AstraLux programme, surveying hundreds of M-dwarf systems for multiplicity and obtaining astrometric epochs for orbital constraints. Our new VLT/SINFONI data provides resolved spectral type classifications in the J, H, and K bands for seven of these low-mass M-dwarf binaries, which we determine by comparing them to empirical templates and examining the strength of water absorption in the K band. The medium resolution K-band spectra also allows us to derive effective temperatures for the individual components. All targets in the survey display several signs of youth, and some have kinematics similar to young moving groups, or low surface gravities which we determined from measuring equivalent widths of gravity sensitive alkali lines in the J band. Resolved photometry from our targets is also compared with isochrones from theoretical evolutionary models, further implying young ages. Dynamical masses will be provided from continued monitoring of these systems, which can be seen as emblematic binary benchmarks that may be used to calibrate evolutionary models for low-mass stars in the future.

Context. Brown dwarfs in the spectral range L9-T3.5, within the so called L/T transition, have been shown to be variable at higher amplitudes and with greater frequency than other field dwarfs. This strong variability allows for the probing of their atmospheric structure in 3D through multi-wavelength observations for studying the underlying physical mechanisms responsible for the variability. The few known strongly variable dwarfs in this range have been extensively studied. Now, more variables such as these need to be discovered and studied to better constrain atmospheric models. This is also critical to better understand giant exoplanets and to shed light on a number of possible correlations between brown dwarf characteristics and variability.

Aims. Previous studies suggest an occurrence rate for strong variability (peak-to-peak amplitudes >2%) of up to similar to 39% among brown dwarfs within the L/T transition. In this work, we aim to discover new strong variables in this spectral range by targeting ten previously unsurveyed brown dwarfs.

Methods. We used the NOTCam at the Nordic Optical Telescope to observe 11 targets, with spectral types ranging from L9.5 to T3.5, in the J-band during October 2017 and September 2018. Using differential aperture photometry, we then analysed the light curves for significant variability using Lomb-Scargle periodogram algorithms and least squares fitting.

Results. We report first discoveries of strong and significant variability in four out of the ten targets (false alarm probability <0.1%), measuring peak-to-peak amplitudes up to 10.7 +/- 0.4% in J for the T1 dwarf 2MASS J22153705+2110554, for which we observe significant light curve evolution between the 2017 and 2018 epochs. We also report a marginally significant detection of strong variability, and confirm that the well known 2MASS J01365662+0933473 is still strongly variable three years after the last reported epoch. Finally, we present an extensive multi-epoch catalogue of strong variables reported in the literature and discuss possible correlations that are identifiable from the catalogue.

Conclusions. We significantly add to the number of known strong variables, and through Poisson statistics infer an occurrence rate for strong variability among L9-T3.5 brown dwarfs of 40(-19)(+32)%, which is in agreement with previous estimates. The new variables identified in this work are also excellently suited for extensive multi-wavelength observations dedicated to probing the 3D structure of brown dwarf atmospheres.

The number of supernovae known to be connected with long-duration gamma-ray bursts (GRBs) is increasing and the link between these events is no longer exclusively found at low redshift (z less than or similar to 0.3) but is well established also at larger distances. We present a new case of such a liaison at z = 0.33 between GRB171010A and SN 2017htp. It is the second closest GRB with an associated supernova of only three events detected by Fermi-LAT. The supernova is one of the few higher redshift cases where spectroscopic observations were possible and shows spectral similarities with the well-studied SN 1998bw, having produced a similar Ni mass (M-Ni = 0.33 +/- 0.02 M-circle dot) with slightly lower ejected mass (M-ej = 4.1 +/- 0.7 M-circle dot) and kinetic energy (E-K = 8.1 +/- 2.5 x 10(51) erg). The host-galaxy is bigger in size than typical GRB host galaxies, but the analysis of the region hosting the GRB revealed spectral properties typically observed in GRB hosts and showed that the progenitor of this event was located in a very bright H II region of its face-on host galaxy, at a projected distance of similar to 10 kpc from its galactic centre. The star-formation rate (SFRGRB similar to 0.2 M-circle dot yr(-1)) and metallicity (12 + log(O/H) similar to 8.15 +/- 0.10) of the GRB star-forming region are consistent with those of the host galaxies of previously studied GRB-SN systems.

We observe 14 young low-mass substellar objects using the VLT/SINFONI integral field spectrograph with laser guide star adaptive optics to detect and characterize three candidate binary systems. All three binary candidates show strong signs of youth, with two of them likely belonging to young moving groups. Together with the adopted young-moving-group ages we employ isochrones from the BT-Settle CIFIST substellar evolutionary models to estimate individual masses for the binary components. We find 2MASS J15104786-2818174 to be part of the approximate to 30-50 Myr Argus moving group and to be composed of a 34-48 M-Jup primary brown dwarf with spectral type M9 gamma and a fainter 15 22 MJup companion, separated by approximate to 100 mas. 2MASS J22025794-5605087 is identified as an almost equal-mass binary in the AB Dor moving group, with a projected separation of approximate to 60 mas. Both components share spectral type M9 gamma/beta, which with the adopted age of 120 200 Myr yields masses in the range of 50 68 M-Jup for each component individually. The observations of 2MASS J15474719-2423493 are of lower quality and we obtain no spectral characterization for the target, but resolve two components separated by approximate to 170 mas which with the predicted young field age of 30 50 Myr yields individual masses below 20 M-Jup. Out of the three candidate binary systems, 2MASS J22025794-5605087 has unambiguous spectroscopic signs of being a bona-fide binary, while the other two will require second-epoch confirmation. The small projected separations between the binary components correspond to physical separations of approximate to 4-7AU, meaning that astrometric monitoring of just a few years would be adequate to generate constrained orbital fits and dynamical masses for the systems. In combination with their young ages, these binaries will prove to be excellent benchmarks for calibrating substellar evolutionary models down to a very low-mass regime.

Stars are dispersed all over the sky within our Galaxy, appearing in large varieties of ages and sizes. How- ever, estimating said traits proves not to always be trivial, but certain circumstances allow us to probe the characteristics of stellar binaries. Fortunately, most stars are found to be part of binary or multiple systems, and through their brightness we can study their sizes, while their dynamical interactions let us derive masses. Although absolute ages are near-impossible to estimate, we posses several methods for constraining age-limits of stars at various evolutionary stages. Theoretical models can provide us with an idea of some of the attributes of stars, but sometimes require assumptions that are not always validated. When fundamental physical laws can be used to deduce individual masses of stellar systems, we procure means to constrain and calibrate those models. As such, the multiplicity of different types of stars is of high importance, which separation and mass distributions can also help us constrain formation scenarios among the different mass-regimes. In this review we compile information regarding the detection of binary and multiple stellar systems, as well as how one can go about to estimate their most fundamental quantities including mass and age. We also compare the different methods and techniques employed throughout the literature, addressing various caveats, examples of usage and the future outlook with coming improvements. In the final part of the review, we present the results from a recent paper about the orbital motions of a triple system, which yield a dynamical mass of the system which shows inconsistencies with the theoretical mass obtained from current low-mass stellar evolutionary models.

Stjärnor är utspridda över hela himlen inom vår galax, och förekommer i flera variationer av åldrar och storlekar. Att uppskatta dessa egenskaper visar sig emellertid inte alltid vara trivialt. Det finns dock omständigheter som tillåter oss att undersöka karaktärsdragen hos binära stjärnor. Lyckligtvis så tillhör de flesta stjärnor binär- eller multipelsystem, och genom deras ljusstyrka kan vi studera deras storlekar, medan deras dynamiska interaktioner tillåter oss härleda massor. Även om absoluta åldrar är näst intill omöjliga att bestämma så har vi flera metoder för att sätta begränsningar på åldern hos stjärnor vid olika evolutionssteg. Teoretiska modeller kan ge oss en uppfattning om en del av attributen hos stjärnor, men kräver ibland osäkra antaganden. När fundamentala fysiska lagar kan användas för att bestämma individuella massor hos stjärnsystem kan vi använda dessa för att kalibrera de teoretiska modellerna. Därför är mångfalden hos stjärnor av stor betydelse, där även fördelningarna av separation och massor hos stjärnsystemen kan hjälpa oss förstå och reglera bildningsscenarion inom de olika massgränserna. I den här avhandlingen samlar vi information om hur binära- och multipel-stjärnsystem kan upptäckas, men visar även hur en kan gå tillväga för att bestämma deras mest fundamentala egenskaper såsom massa och ålder. Vi jämför också de olika metoder och tekniker som används i litteraturen, påpekar de olika problem som uppstår, samt ger exempel på användning och de framtidsutsikter som förväntas med uppkommande förbättringar. I den sista delen av avhandlingen presenterar vi resultat från en nyligen publicerad artikel angående omloppsbanorna i ett trippel-stjärnsystem som ger en dynamisk massa för systemet som motsäger den teoretiska massan uppskattad från rådande teoretiska modeller för lågmassiva stjärnor.

The relationship between luminosity and mass is of fundamental importance for direct imaging studies of brown dwarf and planetary companions to stars. In principle this can be inferred from theoretical mass-luminosity models; however, these relations have not yet been thoroughly calibrated, since there is a lack of substellar companions for which both the brightness and mass have been directly measured. One notable exception is GJ 758 B, a brown dwarf companion in a similar to 20 AU orbit around a nearby Sun-like star, which has been both directly imaged and dynamically detected through a radial velocity trend in the primary. This has enabled a mass constraint for GJ 758 B of 42(-7)(+19) M-Jup. Here, we note that Gaia is ideally suited for further constraining the mass of intermediate-separation companions such as GJ 758 B. A study of the differential proper motion, Delta mu, with regards to HIPPARCOS is particularly useful in this context, as it provides a long time baseline for orbital curvature to occur. By exploiting already determined orbital parameters, we show that the dynamical mass can be further constrained to 42.4(-5.0)(+5.6) M-Jup through the Gaia-HIPPARCOS Delta mu motion. We compare the new dynamical mass estimate with substellar evolutionary models and confirm previous indications that there is significant tension between the isochronal ages of the star and companion, with a preferred stellar age of <= 5 Gyr while the companion is only consistent with very old ages of >= 8 Gyr.

Binary stars constitute a large percentage of the stellar population, yet relatively little is known about the planetary systems orbiting them. Most constraints on circumbinary planets (CBPs) so far come from transit observations with the Kepler telescope, which is sensitive to close-in exoplanets but does not constrain planets on wider orbits. However, with continuous developments in high-contrast imaging techniques, this population can now be addressed through direct imaging. We present the full survey results of the Search for Planets Orbiting Two Stars (SPOTS) survey, which is the first direct imaging survey targeting CBPs. The SPOTS observational program comprises 62 tight binaries that are young and nearby, and thus suitable for direct imaging studies, with VLT/NaCo and VLT/SPHERE. Results from SPOTS include the resolved circumbinary disk around AK Sco, the discovery of a low-mass stellar companion in a triple packed system, the relative astrometry of up to 9 resolved binaries, and possible indications of non-background planetary-mass candidates around HIP 77911. We did not find any CBP within 300 AU, which implies a frequency upper limit on CBPs (1-15 M-Jup) of 6-10% between 30-300 AU. Coupling these observations with an archival dataset for a total of 163 stellar pairs, we find a best-fit CBP frequency of 1.9% (2-15 M-Jup) between 1 and 300 AU with a 10.5% upper limit at a 95% confidence level. This result is consistent with the distribution of companions around single stars.

We combine new Lucky Imaging astrometry from New Technology Telescope /AstraLux Sur with already published astrometry from the AstraLux Large M-dwarf Multiplicity Survey to compute orbital elements and individual masses of the 2MASS J10364483 + 1521394 triple system belonging to the Ursa-Major moving group. The system consists of one primary low-mass M-dwarf orbited by two less massive companions, for which we determine a combined dynamical mass of MB+C = 0 : 48 +/- 0 : 14 M-circle dot. We show from the companions' relative motions that they are of equal mass (with a mass ratio of 1 : 00 +/- 0 : 03), thus 0 : 24 +/- 0 : 07 M-circle dot individually, with a separation of 3 : 2 +/- 0 : 3 AU, and we conclude that these masses are significantly higher (30%) than what is predicted by theoretical stellar evolutionary models. The biggest uncertainty remains the distance to the system, here adopted as 20 : 1 +/- 2 : 0 pc based on trigonometric parallax, whose ambiguity has a major impact on the result. With the new observational data we are able to conclude that the orbital period of the BC pair is 8.4(-0.021)(+0.04) yr.