We present the results of long-baseline optical interferometry observations using the Precision Astronomical Visual Observations (PAVO) beam combiner at the Center for High Angular Resolution Astronomy (CHARA) Array to measure the angular sizes of three bright Kepler stars: θ Cygni, and both components of the binary system 16 Cygni. Supporting infrared observations were made with the Michigan Infrared Combiner (MIRC) and Classic beam combiner, also at the CHARA Array. We find limb-darkened angular diameters of 0.753 ± 0.009 mas for θ Cyg, 0.539 ± 0.007 mas for 16 Cyg A and 0.490 ± 0.006 mas for 16 Cyg B. The Kepler Mission has observed these stars with outstanding photometric precision, revealing the presence of solar-like oscillations. Due to the brightness of these stars the oscillations have exceptional signal-to-noise, allowing for detailed study through asteroseismology, and are well constrained by other observations. We have combined our interferometric diameters with Hipparcos parallaxes, spectrophotometric bolometric fluxes and the asteroseismic large frequency separation to measure linear radii (θ Cyg: 1.48±0.02 R ⊙ , 16 Cyg A: 1.22±0.02 R ⊙ , 16 Cyg B: 1.12±0.02 R ⊙ ), effective temperatures (θ Cyg: 6749±44 K, 16 Cyg A: 5839±42 K, 16 Cyg B: 5809±39 K), and masses (θ Cyg: 1.37±0.04 M ⊙ , 16 Cyg A: 1.07±0.05 M ⊙ , 16 Cyg B: 1.05±0.04 M ⊙ ) for each star with very little model dependence. The measurements presented here will provide strong constraints for future stellar modelling efforts.

Abstract. Cepheids play a key role in astronomy as standard candles for measuring intergalactic distances. Their distance is usually inferred from the period-luminosity relationship, calibrated using the semi-empirical Baade-Wesselink method. Using this method, the distance is known to a multiplicative factor, called the projection factor. Presently, this factor is computed using numerical models -it has hitherto never been measured directly. Based on our new interferometric measurements obtained with the CHARA Array and the already published parallax, we present a geometrical measurement of the projection factor of a Cepheid, δ Cep. The value we determined, p = 1.27 ± 0.06, confirms the generally adopted value of p = 1.36 within 1.5 sigmas. Our value is in line with recent theoretical predictions of Nardetto et al. (2004, A&A, 428, 131).

Epsilon Aurigae (epsilon Aur) is a visually bright, eclipsing binary star system with a period of 27.1 years. The cause of each 18-month-long eclipse has been a subject of controversy for nearly 190 years because the companion has hitherto been undetectable. The orbital elements imply that the opaque object has roughly the same mass as the visible component, which for much of the last century was thought to be an F-type supergiant star with a mass of approximately 15M[symbol:see text] (M[symbol:see text], mass of the Sun). The high mass-to-luminosity ratio of the hidden object was originally explained by supposing it to be a hyperextended infrared star or, later, a black hole with an accretion disk, although the preferred interpretation was as a disk of opaque material at a temperature of approximately 500 K, tilted to the line of sight and with a central opening. Recent work implies that the system consists of a low-mass (2.2M[symbol:see text]-3.3M[symbol:see text]) visible F-type star, with a disk at 550 K that enshrouds a single B5V-type star. Here we report interferometric images that show the eclipsing body moving in front of the F star. The body is an opaque disk and appears tilted as predicted. Adopting a mass of 5.9M[symbol:see text] for the B star, we derive a mass of approximately (3.6 +/- 0.7)M[symbol:see text] for the F star. The disk mass is dynamically negligible; we estimate it to contain approximately 0.07M[symbol:see text] (M[symbol:see text], mass of the Earth) if it consists purely of dust.

Context. The quest for hot dust in the central region of debris disks requires high resolution and high dynamic range imaging. Nearinfrared interferometry is a powerful means to directly detect faint emission from hot grains. Aims. We probed the first 3 AU around τ Ceti and Eridani with the CHARA array (Mt Wilson, USA) in order to gauge the 2 µm excess flux emanating from possible hot dust grains in the debris disks and to also resolve the stellar photospheres. Methods. High precision visibility amplitude measurements were performed with the FLUOR single mode fiber instrument and telescope pairs on baselines ranging from 22 to 241 m of projected length. The short baseline observations allow us to disentangle the contribution of an extended structure from the photospheric emission, while the long baselines constrain the stellar diameter. Results. We have detected a resolved emission around τ Cet, corresponding to a spatially integrated, fractional excess flux of 0.98 ± 0.21 × 10 −2 with respect to the photospheric flux in the K -band. Around Eri, our measurements can exclude a fractional excess of greater than 0.6 × 10 −2 (3σ). We interpret the photometric excess around τ Cet as a possible signature of hot grains in the inner debris disk and demonstrate that a faint, physical or background, companion can be safely excluded. In addition, we measured both stellar angular diameters with an unprecedented accuracy: Θ LD (τ Cet) = 2.015 ± 0.011 mas and Θ LD ( Eri) = 2.126 ± 0.014 mas.

Wolf-Rayet (WR) stars represent one of the final stages of massive stellar evolution. Relatively little is known about this short-lived phase and we currently lack reliable mass, distance, and binarity determinations for a representative sample. Here we report the first visual orbit for WR 140 (= HD193793), a WC7 + O5 binary system known for its periodic dust production episodes triggered by intense colliding winds near periastron passage. The Infrared-Optical Telescope Array and Center for High Angular Resolution Astronomy interferometers resolved the pair of stars in each year from 2003 to 2009, covering most of the highly eccentric, 7.9 year orbit. Combining our results with the recently improved double-line spectroscopic orbit of Fahed et al., we find the WR 140 system is located at a distance of 1.67 ± 0.03 kpc, composed of a WR star with M WR = 14.9 ± 0.5 M and an O star with M O = 35.9 ± 1.3 M . Our precision orbit yields key parameters with uncertainties ∼6× smaller than previous work and paves the way for detailed modeling of the system. Our newly measured flux ratios at the near-infrared H and Ks bands allow a spectral energy distribution decomposition and analysis of the component evolutionary states.