Hypervelocity stars (HVS) travel with velocities so high, that they exceed the escape velocity of the . Several acceleration mechanisms have been discussed. Only one HVS (US 708, HVS 2) is a compact helium star (2). Here we present a spectroscopic and kinematic analysis of US 708. Travelling with a velocity of ∼ 1200 km s −1 , it is the fastest unbound star in our Galaxy. In reconstructing its trajectory, the Galactic center becomes very unlikely as an origin, which is hardly consistent with the most favored ejection mechanism for the other HVS. Furthermore, we discovered US 708 to be a fast rotator.According to our binary evolution model it was spun-up by tidal interaction in a close binary and is likely to be the ejected donor remnant of a thermonuclear supernova.According to the widely-accepted theory for the acceleration of HVS, a close binary is disrupted by the supermassive black hole (SMBH) in the centre of our Galaxy and one component is ejected as hypervelocity star (4). In an alternative scenario US 708 was proposed to be ejected from an ultra-compact binary star by a thermonuclear supernova type Ia (SN Ia) (5). However, previous observational evidence was insufficient to put firm constraints on its past evolution.Here we show that US 708 is the fastest unbound star in our Galaxy, provide evidence for the supernova ejection scenario and identify a progenitor population of SN Ia.In contrast to all other known HVS US 708 has been classified as hot subdwarf star (sdO/B).Those stars are evolved, core helium-burning objects with low masses around 0.5 M ⊙ . About half of the sdB stars reside in close binaries with periods ranging from ∼ 0.1 d to ∼ 30 d (6,7). The hot subdwarf is regarded as the core of a former red giant star that has been stripped off almost all of its hydrogen envelope through interaction with a close companion star (8,9).Apparently single hot subdwarf stars like US 708 itself are known as well. However, binary evolution has also been proposed in this case, since the merger of two helium white dwarfs

Type Ia supernovae (SN Ia) are the most important standard candles for measuring the expansion history of the universe. The thermonuclear explosion of a white dwarf can explain their observed properties, but neither the progenitor systems nor any stellar remnants have been conclusively identified. Underluminous SN Ia have been proposed to originate from a so-called double-detonation of a white dwarf. After a critical amount of helium is deposited on the surface through accretion from a close companion, the helium is ignited causing a detonation wave that triggers the explosion of the white dwarf itself. We have discovered both shallow transits and eclipses in the tight binary system CD-30 • 11223 composed of a carbon/oxygen white dwarf and a hot helium star, allowing us to determine its component masses and fundamental parameters. In the future the system will transfer mass from the helium star to the white dwarf. Modelling this process we find that the detonation in the accreted helium layer is sufficiently strong to trigger the explosion of the core. The helium star will then be ejected at such high velocity that it will escape the Galaxy. The predicted properties of this remnant are an excellent match to the so-called hypervelocity star US 708, a hot, helium-rich star moving at more than 750 km s −1 , sufficient for it to leave the Galaxy. The identification of both progenitor and remnant provides a consistent picture of the formation and evolution of underluminous SNIa.

Hot subdwarf B stars (sdBs) are core helium-burning stars located on the extreme horizontal branch. About half of the known sdB stars are found in close binaries. Their short orbital periods of 1.2 h to a few days suggest that they are post common-envelope systems. Eclipsing hot subdwarf binaries are rare but are important in determining the fundamental stellar parameters. Low-mass companions are identified by the reflection effect. In most cases, the companion is a main sequence star near the stellar mass limit. Here, we report the discovery of an eclipsing hot subdwarf binary SDSS J162256.66+473051.1 (J1622) with very short orbital period (0.0697 d), which has been found in the course of the MUCHFUSS project. The lightcurve shows grazing eclipses and a prominent reflection effect. An analysis of the light-and radial velocity (RV) curves indicated a mass ratio of q = 0.1325, an RV semi-amplitude K = 47.2 km s −1 , and an inclination of i = 72.33 • . We show that a companion mass of 0.064 M , which is well below the hydrogenburning limit, is the most plausible solution, which implies a mass close to the canonical mass (0.47 M ) of the sdB star. Therefore, the companion is a brown dwarf, which has not only survived the engulfment by the red-giant envelope but also triggered its ejection and enabled the sdB star to form. The rotation of J1622 is expected to be tidally locked to the orbit. However, J1622 rotates too slowly (v rot = 74.5 ± 7 km s −1 ) to be synchronized, challenging tidal interaction models.

The project Massive Unseen Companions to Hot Faint Underluminous Stars from SDSS (MUCHFUSS) aims to find sdBs with compact companions such as massive white dwarfs, neutron stars, or black holes. Here we provide classifications, atmospheric parameters, and a complete radial velocity (RV) catalogue containing 1914 single measurements for a sample of 177 hot subluminous stars discovered based on SDSS DR7; 110 stars show significant RV variability, while 67 qualify as candidates. We constrain the fraction of close massive compact companions of hydrogen-rich hot subdwarfs in our sample to be smaller than ∼1.3%, which is already close to the theoretical predictions. However, the sample might still contain such binaries with longer periods exceeding ∼8 d. We detect a mismatch between the ΔRV max -distribution of the sdB and the more evolved sdOB and sdO stars, which challenges our understanding of their evolutionary connection. Furthermore, irregular RV variations of unknown origin with amplitudes of up to ∼180 km s −1 on timescales of years, days, and even hours have been detected in some He-sdO stars. They might be connected to irregular photometric variations in some cases.

Eclipsing post-common envelope binaries are highly important for resolving the poorly understood, very short-lived common envelope phase of stellar evolution. Most hot subdwarfs (sdO/Bs) are the bare helium-burning cores of red giants which have lost almost all of their hydrogen envelopes. This mass loss is often triggered by common envelope interactions with close stellar or even sub-stellar companions. Cool companions to hot subdwarf stars such as late-type stars and brown dwarfs are detectable from characteristic light curve variations -reflection effects and often eclipses. In the recently published catalog of eclipsing binaries in the Galactic Bulge and in the ATLAS (Asteroid Terrestrial-impact Last Alert System) survey, we discovered 125 new eclipsing systems showing a reflection effect by visual inspection of the light curves and using a machine-learning algorithm, in addition to the 36 systems discovered before by the OGLE (Optical Gravitational Lesing Experiment) team. The EREBOS (Eclipsing Reflection Effect Binaries from Optical Surveys) project aims at analyzing all newly discovered eclipsing binaries of the HW Vir type (hot subdwarf + close, cool companion) based on a spectroscopic and photometric follow up to derive the mass distribution of the companions, constrain the fraction of sub-stellar companions and determine the minimum mass needed to strip off the red-giant envelope. To constrain the nature of the primary we derived the absolute magnitude and the reduced proper motion of all our targets with the help of the parallaxes and proper motions measured by the Gaia mission and compared those to the Gaia white dwarf candidate catalogue. For a sub-set of our targets with observed spectra the nature could be derived by measuring the atmospheric parameter of the primary confirming that less than 10% of our systems are not sdO/Bs with cool companions but white dwarfs or central stars of planetary nebula. This large sample of eclipsing hot subdwarfs with cool companions allowed us to derive a significant period distribution for hot subdwarfs with cool companions for the first time showing that the period distribution is much broader than previously thought and ideally suited to find the lowest mass companions to hot subdwarf stars. The comparison with related binary populations shows that the period distribution of HW Vir systems is very similar to WD+dM systems and central stars of planetary nebula with cool companions. In the future several new photometric surveys will be carried out, which will increase the sample of this project even more giving the potential to test many aspects of common envelope theory and binary evolution.