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In the era of precision cosmology, it is essential to determine the Hubble constant empirically with an accuracy of one per cent or better 1 . At present, the uncertainty on this constant is dominated by the uncertainty in the calibration of the Cepheid period–luminosity relationship 2 , 3 (also known as the Leavitt law). The Large Magellanic Cloud has traditionally served as the best galaxy with which to calibrate Cepheid period–luminosity relations, and as a result has become the best anchor point for the cosmic distance scale 4 , 5 . Eclipsing binary systems composed of late-type stars offer the most precise and accurate way to measure the distance to the Large Magellanic Cloud. Currently the limit of the precision attainable with this technique is about two per cent, and is set by the precision of the existing calibrations of the surface brightness–colour relation 5 , 6 . Here we report a calibration of the surface brightness–colour relation with a precision of 0.8 per cent. We use this calibration to determine a geometrical distance to the Large Magellanic Cloud that is precise to 1 per cent based on 20 eclipsing binary systems. The final distance is 49.59 ± 0.09 (statistical) ± 0.54 (systematic) kiloparsecs. A new calibration of the surface brightness–colour relation of eclipsing binary stars gives a distance to the Large Magellanic Cloud that is precise to one per cent.

We report the discovery of ASASSN-15lh (SN 2015L), which we interpret as the most luminous supernova yet found. At redshift z = 0.2326, ASASSN-15lh reached an absolute magnitude of Mu,AB = −23.5 ± 0.1 and bolometric luminosity Lbol = (2.2 ± 0.2) × 10⁴⁵ ergs s⁻¹, which is more than twice as luminous as any previously known supernova. It has several major features characteristic of the hydrogen-poor super-luminous supernovae (SLSNe-l), whose energy sources and progenitors are currently poorly understood. In contrast to most previously known SLSNe-l that reside in star-forming dwarf galaxies, ASASSN-15lh appears to be hosted by a luminous galaxy (MK ≈ −25.5) with little star formation. In the 4 months since first detection, ASASSN-15lh radiated (1.1 ± 0.2) × 10⁵² ergs, challenging the magnetar model for its engine.

Cepheid variable mass Cepheid variable stars have been important in the development of modern astrophysics through their use in establishing cosmic distances, but despite extensive research they retain some of their mysteries. One is the mass discrepancy problem, the fact that the masses of classical Cepheid supergiants calculated from pulsation theory (it is pulsation that causes their luminosity to vary) are smaller than the masses calculated from stellar evolution models. The ideal system in which to make an accurate mass determination would be a well-detached double-lined eclipsing binary in which one of the components was a classical Cepheid. Pietrzynski et al . report the discovery of just such a system in the Large Magellanic Cloud. The resultant mass determination, to a precision of one per cent, is in agreement with the mass as predicted by pulsation theory. Masses of pulsating classical Cepheid supergiants derived from stellar pulsation theory are smaller than the masses derived from stellar evolution theory. An independent determination for a classical Cepheid in a binary system is needed to determine which is correct. These authors report the discovery of a classical Cepheid in the Large Magellanic Cloud. They determine the mass to a precision of one per cent and show that it agrees with its pulsation mass. Stellar pulsation theory provides a means of determining the masses of pulsating classical Cepheid supergiants—it is the pulsation that causes their luminosity to vary. Such pulsational masses are found to be smaller than the masses derived from stellar evolution theory: this is the Cepheid mass discrepancy problem 1 , 2 , for which a solution is missing 3 , 4 , 5 . An independent, accurate dynamical mass determination for a classical Cepheid variable star (as opposed to type-II Cepheids, low-mass stars with a very different evolutionary history) in a binary system is needed in order to determine which is correct. The accuracy of previous efforts to establish a dynamical Cepheid mass from Galactic single-lined non-eclipsing binaries was typically about 15–30% (refs 6 , 7 ), which is not good enough to resolve the mass discrepancy problem. In spite of many observational efforts 8 , 9 , no firm detection of a classical Cepheid in an eclipsing double-lined binary has hitherto been reported. Here we report the discovery of a classical Cepheid in a well detached, double-lined eclipsing binary in the Large Magellanic Cloud. We determine the mass to a precision of 1% and show that it agrees with its pulsation mass, providing strong evidence that pulsation theory correctly and precisely predicts the masses of classical Cepheids.

Most of the progenitors of type Ia supernovae in nearby spiral galaxies may be white dwarf−normal star binary systems. Type Ia supernovae are key tools for measuring distances on a cosmic scale. They are generally thought to be the thermonuclear explosion of an accreting white dwarf in a close binary system. The nature of the mass donor is still uncertain. In the single-degenerate model it is a main-sequence star or an evolved star, whereas in the double-degenerate model it is another white dwarf. We show that the velocity structure of absorbing material along the line of sight to 35 type Ia supernovae tends to be blueshifted. These structures are likely signatures of gas outflows from the supernova progenitor systems. Thus, many type Ia supernovae in nearby spiral galaxies may originate in single-degenerate systems.

The pulsating star OGLE-BLG-RRLYR-02792 is known to be a member of an eclipsing binary system, and its mass is now determined to be only 0.26 times that of the Sun, meaning that it cannot be a classical RR Lyrae pulsator. A new class of variable star: not RR Lyrae Astronomers use pulsating variable stars of the RR Lyrae type as indicators of the ages of galaxies, and as tools to measure distances to nearby galaxies. So the news that one of these stars had apparently been found as part of an eclipsing binary system was welcome: it meant that the mass of one of these pulsators, previously available only from models, could be unambiguously determined. But the story is not that simple. Pietrzyński et al . have now determined that the star in question, known as RRLYR-02792, has a mass 0.26 times that of the Sun. This means that it is not a classical RR Lyrae star. Instead, it seems to be a pulsator with observational properties temporarily similar to those of classical RR Lyrae stars, but with different stellar parameters and a different evolutionary history as part of a close binary. The authors estimate that 0.2% of samples of RR Lyrae variables may by contaminated by systems similar to this one, so distances previously measured using RR Lyrae stars should not be significantly affected by the presence of these binaries. RR Lyrae pulsating stars have been extensively used as tracers of old stellar populations for the purpose of determining the ages of galaxies, and as tools to measure distances to nearby galaxies 1 , 2 , 3 . There was accordingly considerable interest when the RR Lyrae star OGLE-BLG-RRLYR-02792 (referred to here as RRLYR-02792) was found to be a member of an eclipsing binary system 4 , because the mass of the pulsator (hitherto constrained only by models) could be unambiguously determined. Here we report that RRLYR-02792 has a mass of 0.26 solar masses ( ) and therefore cannot be a classical RR Lyrae star. Using models, we find that its properties are best explained by the evolution of a close binary system that started with and stars orbiting each other with an initial period of 2.9 days. Mass exchange over 5.4 billion years produced the observed system, which is now in a very short-lived phase where the physical properties of the pulsator happen to place it in the same instability strip of the Hertzsprung–Russell diagram as that occupied by RR Lyrae stars. We estimate that only 0.2 per cent of RR Lyrae stars may be contaminated by systems similar to this one, which implies that distances measured with RR Lyrae stars should not be significantly affected by these binary interlopers.

We report the discovery of ASASSN-15lh (SN 2015L), which we interpret as the most luminous supernova yet found. At redshift z = 0.2326, ASASSN-15lh reached an absolute magnitude of Mu ,AB = -23.5 ± 0.1 and bolometric luminosity Lbol = (2.2 ± 0.2) × 10(45) ergs s(-1), which is more than twice as luminous as any previously known supernova. It has several major features characteristic of the hydrogen-poor super-luminous supernovae (SLSNe-I), whose energy sources and progenitors are currently poorly understood. In contrast to most previously known SLSNe-I that reside in star-forming dwarf galaxies, ASASSN-15lh appears to be hosted by a luminous galaxy (MK ≈ -25.5) with little star formation. In the 4 months since first detection, ASASSN-15lh radiated (1.1 ± 0.2) × 10(52) ergs, challenging the magnetar model for its engine.

Observations of eight long-period, late-type eclipsing-binary systems composed of cool, giant stars are used to determine a distance to the Large Magellanic Cloud accurate to 2.2 per cent, providing a base for a determination of the Hubble constant to an accuracy of 3 per cent. Accurate distance to our nearest-neighbour galaxy The physical properties of stars in eclipsing binary systems can be accurately determined thanks to the intimate interactions between the two bodies, and by monitoring the fluctuating light from such systems it is possible to obtain accurate extragalactic distance measurement. This technique has now been used to determine the most accurate distance estimate yet for the Large Magellanic Cloud (LMC), our nearest-neighbour galaxy. The data from eight long-period, late-type eclipsing systems particularly suitable for this calibration technique suggest that the LMC is around 49.97 kiloparsecs from us, to an accuracy of 2.2%. The distance to the LMC is a key element in determining the Hubble constant, an important measure of the rate of expansion of the Universe. In the era of precision cosmology, it is essential to determine the Hubble constant to an accuracy of three per cent or better 1 , 2 . At present, its uncertainty is dominated by the uncertainty in the distance to the Large Magellanic Cloud (LMC), which, being our second-closest galaxy, serves as the best anchor point for the cosmic distance scale 2 , 3 . Observations of eclipsing binaries offer a unique opportunity to measure stellar parameters and distances precisely and accurately 4 , 5 . The eclipsing-binary method was previously applied to the LMC 6 , 7 , but the accuracy of the distance results was lessened by the need to model the bright, early-type systems used in those studies. Here we report determinations of the distances to eight long-period, late-type eclipsing systems in the LMC, composed of cool, giant stars. For these systems, we can accurately measure both the linear and the angular sizes of their components and avoid the most important problems related to the hot, early-type systems. The LMC distance that we derive from these systems (49.97 ± 0.19 (statistical) ± 1.11 (systematic) kiloparsecs) is accurate to 2.2 per cent and provides a firm base for a 3-per-cent determination of the Hubble constant, with prospects for improvement to 2 per cent in the future.

Type la supernovae are key tools for measuring distances on a cosmic scale. They are generally thought to be the thermonuclear explosion of an accreting white dwarf in a close binary system. The nature of the mass donor is still uncertain. In the single-degenerate model it is a main-sequence star or an evolved star, whereas in the double-degenerate model it is another white dwarf. We show that the velocity structure of absorbing material along the line of sight to 35 type la supernovae tends to be blueshifted. These structures are likely signatures of gas outflows from the supernova progenitor systems. Thus, many type la supernovae in nearby spiral galaxies may originate in single-degenerate systems.

Our goal is to determine, with high accuracy, the physical and orbital parameters of two double-lined eclipsing binary systems, where the components are two giant stars. We also aim to study the evolutionary status of the binaries, to derive the distances towards them by using a surface brightness-colour relation, and to compare these measurements with the measurements presented by the Gaia mission. In order to measure the physical and orbital parameters of the systems, we analysed the light curves and radial-velocity curves with the Wilson-Devinney code. We used V band and I-band photometry from the OGLE catalogue and near-infrared photometry obtained with the New Technology Telescope (NTT) equipped with the SOFI instrument. The spectroscopic data were collected with the HARPS spectrograph mounted at the ESO 3.6m telescope and the MIKE spectrograph mounted at the 6.5m Clay telescope. We present the first analysis of this kind for two evolved eclipsing binary systems from the OGLE catalogue: OGLE-BLG-ECL-305487 and OGLE-BLG-ECL-116218. The masses of the components of OGLE-BLG-ECL-305487 are \\(M_1\\) = 1.059 \\(\\pm\\) 0.019 and \\(M_2\\) = 0.991 \\(\\pm\\) 0.018 \\(M_\\odot\\), and the radii are \\(R_1\\) = 19.27 \\(\\pm\\) 0.28 and \\(R_2\\) = 29.99 \\(\\pm\\) 0.24 R\\(_\\odot\\). For OGLE-BLG-ECL-116218, the masses are \\(M_1\\)= 0.969 \\(\\pm\\) 0.012 and \\(M_2\\)= 0.983 \\(\\pm\\) 0.012 \\(M_\\odot\\), while the radii are \\(R_1\\)= 16.73 \\(\\pm\\) 0.28 and \\(R_2\\)= 22.06 \\(\\pm\\) 0.26 \\(R_\\odot\\). The evolutionary status of the systems is discussed based on the PARSEC and MIST isochrones. The ages of the systems were established to be between 7.3-10.9 Gyr for OGLE-BLG-ECL-305487 and around 10 Gyr for OGLE-BLG-ECL-116218. We also measured the distances to the binaries. For OGLE-BLG-ECL-305487, \\(d\\)= 7.80 \\(\\pm\\) 0.18 (stat.) \\(\\pm\\) 0.19 (syst.) kpc and for OGLE-BLG-ECL-116218, \\(d\\)= 7.57 \\(\\pm\\) 0.28 (stat.) \\(\\pm\\) 0.19 (syst.) kpc.

The field of the globular cluster M10 (NGC 6254) was monitored between 1998 and 2015 in a search for variable stars. V -light curves were derived for 40 variables or likely variables, most of which are new detections. Proper motions obtained within the CASE project indicate that 18 newly detected variables and 14 previously known ones are members or likely members of the cluster, including one RRc-type, three type II Cepheids, and 14 SX Phe-type pulsators, one contact binary, and six semi-regular red giants. As a byproduct of the search we discovered a candidate binary comprised of main sequence stars with the record-short orbital period of 0.042 d. We also confirmed the photometric variability of the red straggler M10-VLA1 hinted at by Shishkovsky et al. (2018), who discovered this object spectroscopically. In Appendix 1 we show that CASE proper motion measurements are in a good agreement with those retrieved from the Gaia archive, while Appendix 2 presents evidence for low frequency {\\gamma} Doradus-type oscillations in SX Phe stars belonging to M10.