The project Massive Unseen Companions to Hot Faint Underluminous Stars from SDSS (MUCHFUSS) aims at finding sdBs with compact companions like supermassive white dwarfs (M > 1.0 M ), neutron stars or black holes. The existence of such systems is predicted by binary evolution theory and recent discoveries indicate that they are likely to exist in our Galaxy. A determination of the orbital parameters is sufficient to put a lower limit on the companion mass by calculating the binary mass function. If this lower limit exceeds the Chandrasekhar mass and no sign of a companion is visible in the spectra, the existence of a massive compact companion is proven without the need for any additional assumptions. We identified about 1100 hot subdwarf stars from the SDSS by colour selection and visual inspection of their spectra. Stars with high velocities have been reobserved and individual SDSS spectra have been analysed. In total 127 radial velocity variable subdwarfs have been discovered. Binaries with high RV shifts and binaries with moderate shifts within short timespans have the highest probability of hosting massive compact companions. Atmospheric parameters of 69 hot subdwarfs in these binary systems have been determined by means of a quantitative spectral analysis. The atmospheric parameter distribution of the selected sample does not differ from previously studied samples of hot subdwarfs. The systems are considered the best candidates to search for massive compact companions by follow-up time resolved spectroscopy.

Context. High-velocity stars in the Galactic halo, e.g. the so-called hyper-velocity stars (HVS), are important tracers of the properties of the dark matter halo, in particular its mass. Aims. A search for the fastest stars among hot subdwarfs (sdB) in the halo is carried out to identify HVS, unbound to the Galaxy, and bound population II stars, in order to derive a lower limit to the halo mass. Methods. Based on the SDSS DR6 spectral database we selected stars with high rest-frame velocities. These radial velocity (RV) measurements were verified at several telescopes to exclude RV variable stars. Out of 88 stars observed in the follow-up campaign, 39 stars were found to have constant RVs. For twelve of them we measured a proper motion significantly different from zero and obtained spectroscopic distances from quantitative spectral analysis to construct the full 6D phase space information for a kinematical study.Results. All but one programme sdB show halo characteristics, but can be distinguished as two kinematical groups, one (G1) with the low Galactic rotation typical of halo stars and a second one (G2) with rapid retrograde motion. We also investigate the possibility that the programme stars are not genuine halo stars but ejected from the Galactic disc or bulge. The G1 objects crossed the Galactic plane in the central bulge, whereas the G2 stars did so in the outer Galactic disc. J1211+1437 (G2) is an HVS candidate, as it is unbound to the Galaxy if the standard Galactic potential is adopted. Conclusions. We conclude that in the ejection scenario, G1 stars might have been formed via the slingshot mechanism that invokes acceleration by tidal interaction of a binary with the central supermassive black hole. The G2 stars, however, would originate in the outskirts of the Galactic disc and not in the central bulge. J1211+1437 is the first unbound subdwarf B star, for which we can rule out the slingshot mechanism. Alternatively, we may assume that the stars are old population II stars and so they have to be bound. Then the kinematics of J1211+1437 set a lower limit of 2 × 10 12 M to the mass of the Galactic dark matter halo.

-Hot subdwarf B stars (sdBs) are extreme horizontal branch stars believed to originate from close binary evolution. Indeed about half of the known sdB stars are found in close binaries with periods ranging from a few hours to a few days. The enormous mass loss required to remove the hydrogen envelope of the red-giant progenitor almost entirely can be explained by common envelope ejection. A rare subclass of these binaries are the eclipsing HW Vir binaries where the sdB is orbited by a dwarf M star. Here we report the discovery of an HW Vir system in the course of the MUCHFUSS project. A most likely substellar object (≃ 0.068 M ⊙ ) was found to orbit the hot subdwarf J08205+0008 with a period of 0.096 days. Since the eclipses are total, the system parameters are very well constrained. J08205+0008 has the lowest unambiguously measured companion mass yet found in a subdwarf B binary. This implies that the most likely substellar companion has not only survived the engulfment by the red-giant envelope, but also triggered its ejection and enabled the sdB star to form. The system provides evidence that brown dwarfs may indeed be able to significantly affect late stellar evolution.

Context. Hyper-velocity stars (HVS) are moving so fast that they are unbound to the Galaxy. Dynamical ejection by a supermassive black hole is favoured to explain their origin. Aims. Locating the place of birth of an individual HVS is of utmost importance to understanding the ejection mechanism. Methods. SDSS J013655.91+242546.0 (J0136+2425 for short) was found amongst three high-velocity stars (drawn from a sample of more than 10 000 blue stars), for which proper motions were measured. A kinematical as well as a quantitative NLTE spectral analysis was performed. When combined with the radial velocity (RV) and the spectroscopic distance, the trajectory of the star in the Galactic potential was reconstructed. Results. J0136+2425 is found to be an A-type main-sequence star travelling at ≈590 km s −1 , possibly unbound to the Galaxy and originating in the outer Galactic rim nowhere near the Galactic centre. Conclusions. J0136+2425 is the second HVS candidate with measured proper motion, besides the massive B star HD 271791, and also the second for which its proper motion excludes a Galactic centre origin and, hence, the SMBH slingshot mechanism. Most known HVS are late B-type stars of about 3 M . With a mass of 2.45 M , J0136+2425 resembles a typical HVS far more than HD 271791 does. Hence, this is the first time that a typical HVS is found not to originate in the Galactic centre. Its ejection velocity from the disk is so high (550 km s −1 ) that the extreme supernova binary scenario proposed for HD 271791 is very unlikely.

The project Massive Unseen Companions to Hot Faint Underluminous Stars from SDSS (MUCHFUSS) aims at finding hot subdwarf stars with massive compact companions like massive white dwarfs (M > 1.0 M ), neutron stars or stellar mass black holes. The existence of such systems is predicted by binary evolution theory and recent discoveries indicate that they exist in our Galaxy. First results are presented for seven close binary sdBs with short orbital periods ranging from 0.21 d to 1.5 d. The atmospheric parameters of all objects are compatible with core helium-burning stars. The companions are most likely white dwarfs. In one case the companion could be shown to be a white dwarf by the absence of light-curve variations. However, in most cases late type main sequence stars cannot be firmly excluded. Comparing our small sample with the known population of close sdB binaries we show that our target selection method aiming at massive companions is efficient. The minimum companion masses of all binaries in our sample are high compared to the reference sample of known sdB binaries.