The most powerful tests of stellar models come from the brightest stars in the sky, for which complementary techniques, such as astrometry, asteroseismology, spectroscopy, and interferometry can be combined. The K2 Mission is providing a unique opportunity to obtain high-precision photometric time series for bright stars along the ecliptic. However, bright targets require a large number of pixels to capture the entirety of the stellar flux, and bandwidth restrictions limit the number and brightness of stars that can be observed. To overcome this, we have developed a new photometric technique, that we call halo photometry, to observe very bright stars using a limited number of pixels. Halo photometry is simple, fast and does not require extensive pixel allocation, and will allow us to use K2 and other photometric missions, such as TESS, to observe very bright stars for asteroseismology and to search for transiting exoplanets. We apply this method to the seven brightest stars in the Pleiades open cluster. Each star exhibits variability; six of the stars show what are most-likely slowly pulsating Bstar (SPB) pulsations, with amplitudes ranging from 20 to 2000 ppm. For the star Maia, we demonstrate the utility of combining K2 photometry with spectroscopy and interferometry to show that it is not a 'Maia variable', and to establish that its variability is caused by rotational modulation of a large chemical spot on a 10 d time scale.

We report on interferometric observations with the CHARA Array of two classical Wolf-Rayet stars in suspected binary systems, namely WR 137 and WR 138. In both cases, we resolve the component stars to be separated by a few milliarcseconds. The data were collected in the H-band, and provide a measure of the fractional flux for both stars in each system. We find that the WR star is the dominant H-band light source in both systems ( f WR,137 = 0.59 ± 0.04; f WR,138 = 0.67 ± 0.01), which is confirmed through both comparisons with estimated fundamental parameters for WR stars and O dwarfs, as well as through spectral modeling of each system. Our spectral modeling also provides fundamental parameters for the stars and winds in these systems. The results on WR 138 provide evidence that it is a binary system which may have gone through a previous mass-transfer episode to create the WR star. The separation and position of the stars in the WR 137 system together with previous results from the IOTA interferometer provides evidence that the binary is seen nearly edge-on. The possible edge-on orbit of WR 137 aligns well with the dust production site imaged by the Hubble Space Telescope during a previous periastron passage, showing that the dust production may be concentrated in the orbital plane.

MWC 314 is a bright candidate luminous blue variable that resides in a fairly close binary system, with an orbital period of 60.753±0.003 d. We observed MWC 314 with a combination of optical spectroscopy, broad-band ground-and space-based photometry, as well as with long baseline, near-infrared interferometry. We have revised the singlelined spectroscopic orbit and explored the photometric variability. The orbital light curve displays two minima each orbit that can be partially explained in terms of the tidal distortion of the primary that occurs around the time of periastron. The emission lines in the system are often double-peaked and stationary in their kinematics, indicative of a circumbinary disc. We find that the stellar wind or circumbinary disc is partially resolved in the K ′ -band with the longest baselines of the CHARA Array. From this analysis, we provide a simple, qualitative model in an attempt to explain the observations. From the assumption of Roche Lobe overflow and tidal synchronisation at periastron, we estimate the component masses to be M 1 ≈ 5M ⊙ and M 2 ≈ 15M ⊙ , which indicates a mass of the LBV that is extremely low. In addition to the orbital modulation, we discovered two pulsational modes with the MOST satellite. These modes are easily supported by a low-mass hydrogen-poor star, but cannot be easily supported by a star with the parameters of an LBV. The combination of these results provides evidence that the primary star was likely never a normal LBV, but rather is the product of binary interactions. As such, this system presents opportunities for studying mass-transfer and binary evolution with many observational techniques.

We present interferometric observations of six O-type stars that were made with the Precision Astronomical Visible Observations (PAVO) beam combiner at the Center for High Angular Resolution Astronomy (CHARA) Array. The observations include multiple brackets for three targets, λ Ori A, ζ Oph, and 10 Lac, but there are only preliminary, single observations of the other three stars, ξ Per, α Cam, and ζ Ori A. The stellar angular diameters range from 0.55 milliarcsec for ζ Ori A down to 0.11 mas for 10 Lac, the smallest star yet resolved with the CHARA Array. The rotational oblateness of the rapidly rotating star ζ Oph is directly measured for the first time. We assembled ultraviolet to infrared flux measurements for these stars, and then derived angular diameters and reddening estimates using model atmospheres and an effective temperature set by published results from analysis of the line spectrum. The model-based angular diameters are in good agreement with observed angular diameters. We also present estimates for the effective temperatures of these stars derived by setting the interferometric angular size and fitting the spectrophotometry.

We present interferometric observations of 25 spectral type-B stars that were made with the Precision Astronomical Visible Observations and the CLassic Interferometry with Multiple Baselines beam combiners at the Center for High Angular Resolution Astronomy Array (CHARA). The observations provide the angular sizes of these stars with an average error of 6%. The stars range in size from 1.09 mas for β Tau down to 0.20 mas for 32 Ori. We collected ultraviolet to infrared spectrophotometry and derived temperatures, angular diameters, and reddening estimates that best fit the spectra, as well as solutions with the angular size fixed by the interferometric measurements. There is generally good agreement between the observed and spectral fit angular diameters, indicating that the fluxes predicted from model atmospheres are reliable. On the other hand, the temperatures derived from angular diameters and fluxes tend to be larger (by ≈4%) than those from published results based on analysis of the line spectrum. This discrepancy may in part be attributed to unexplored atmospheric parameters or the existence of unknown companions. The physical radii of the stars are calculated from the angular diameters and Gaia DR2 parallaxes, and the target stars are placed in the Hertzsprung–Russell diagram for comparison with evolutionary tracks.