Long-baseline optical and IR interferometers now routinely resolve the wind and disk-like structures around early-type stars. The typical angular scales resolved by current generation of instruments are well bellow the milli-arcsecond level. These type of observations allow, in some cases for the first time, placing very tight constraints on current theories and models of the circumstellar structures around these type of stars. Specific examples of observations obtained at the Navy Prototype Optical interferometer of the spatially resolved regions around a luminous blue variable star P Cyg and a B-type star with circumstellar disk are presented. The need for connection between interferometric observables and physical parameters predicted by theory and numerical models are emphasized.

We present new physical and orbital parameters of an early-type double-lined eclipsing binary system ALS 1135. The UBVIC light curves and radial velocity curves were modeled simultaneously by means of the Wilson-Devinney code. As a result, we obtained inclination and size of the orbit, as well as masses, radii and effective temperatures of the components.

We have used continuous, high-precision, broadband visible photometry from the MOST satellite to trace wind structures in the WR component of CV Ser over more than a full orbit. Most of the small-scale light-curve variations are likely due to extinction by clumps along the line of sight to the O companion as it orbits and shines through varying columns of the WR wind. Parallel optical spectroscopy from the Mont Megantic Observatory is used to refine the orbital and wind-collision parameters, as well as to reveal line emission from clumps.

Optical and IR spectra, optical to sub-mm photometry, visual imaging polarimetry, and IR high-resolution spectro-interferometry are being used to monitor the new outburst of 28 CMa, which started in 2008 and so far closely resembles previous ones. First modeling based on viscous decretion and focused on constraining the disk viscosity parameter, α, is presented.

We use a combination of VJHK and Spitzer [3.6], [5.8] and [8.0] photometry, to determine IR excesses in a sample of LMC and SMC O stars. This sample is ideal for determining excesses because: 1) the distances to the stars, and hence their luminosities, are well-determined, and; 2) the very small line of sight reddenings minimize the uncertainties introduced by extinction corrections. We find IR excesses much larger than expected from Vink et al. (2001) mass loss rates. This is in contrast to previous wind line analyses for many of the LMC stars which suggest mass loss rates much less than the Vink et al. predictions. Together, these results indicate that the winds of the LMC and SMC O stars are strongly structured (clumped).

We conduct 3D magneto-hydrodynamic (MHD) simulations in order to test the stability of the magnetic equilibrium configuration described by Duez & Mathis (2010). This analytically-derived configuration describes the lowest energy state for a given helicity in a stellar radiation zone. The necessity of taking into account the non force-free property of the large-scale, global field is here emphasized. We then show that this configuration is stable. It therefore provides a useful model to initialize the magnetic topology in upcoming MHD simulations and stellar evolution codes taking into account magneto-rotational transport processes.

Recent space observations with CoRoT and ground-based spectroscopy have shown the presence of different types of pulsations in OB stars. These oscillations could be due to acoustic and gravity modes, solar-like oscillations or even other pulsations of large growth rates. We present a first attempt at interpreting the latter as strange modes.

Hα high resolution spectroscopy combined with detailed numerical models is used to probe the physical conditions, such as density, temperature, and velocity of Be star disks. Models have been constructed for Be stars over a range in spectral types and inclination angles. We find that a variety of line shapes can be obtained by keeping the inclination fixed and changing density alone. This is due to the fact that our models account for disk temperature distributions self-consistently from the requirement of radiative equilibrium. A new analytical tool, called the variability ratio, was developed to identify emission-line stars at particular stages of variability. It is used in this work to quantify changes in the Hα equivalent widths for our observed spectra.

Preliminary results of a four-week multi-colour photometric campaign on previously identified β Cephei stars as well as newly-discovered variable stars in two respective LMC fields are presented. Besides the two targeted β Cephei stars, at least six further presumed B variables are detected. The strongest identified periods appear to lie on the longward end of the galactic β Cep instability strip, as predicted by model calculations.

Since 2003, the amateur astronomical community has decided, in collaboration with the Paris-Meudon Observatory, to coordinate their observations to get the best spectral survey of Be stars as possible. A database for amateur and professional Be star spectra, BeSS, has been created. Spectrographs (up to R=20000) and software tools have been developed for amateurs. Among them, ArasBeam is a web-based tool designed to organize amateur Be spectral observations. A very simple color coding indicates to any observer which stars must be observed on the following night to get the best possible survey of Be stars. So far, more than 11000 amateur spectra have been collected in BeSS. About all bright Be stars (up to magnitude 8) listed in BeSS and visible from the Northern hemisphere have been observed at least one time. In addition, 6 outbursts have been detected by amateurs in the last 2 years.

Observational contraints on the evolution and instabilities of massive stars at very low metallicities are limited. Most of the information come from HST observations of one target, I Zw 18. Recent distance estimates of I Zw 18 put it at 17 Mpc, moving detailed studies of single stars clearly beyond the range of current ground based telescopes. Since massive stars with metallcities of 1/10 of solar and below are our best proxies for massive stars in (proto-) galaxies around the time of reionization, finding them and studying their evolution and instabilities is of premium importance for our understanding of galaxy formation, feedback, and the IGM reionization. Here we present pilot study results of variable stars in two more nearby extremely low metallicity galaxies, UGC 5340 and UGCA 292, and comment on the possibilities of more detailed studies of variable massive stars with new ground-based instrumentation.

The colliding wind binary (CWB) systems η Carinae and WR140 provide unique laboratories for X-ray astrophysics. Their wind-wind collisions produce hard X-rays that have been monitored extensively by several X-ray telescopes, including RXTE. To interpret these RXTE X-ray light curves, we apply 3D hydrodynamic simulations of the wind-wind collision using smoothed particle hydrodynamics (SPH). We find adiabatic simulations that account for the absorption of X-rays from an assumed point source of X-ray emission at the apex of the wind-collision shock cone can closely match the RXTE light curves of both η Car and WR140. This point-source model can also explain the early recovery of η Car's X-ray light curve from the 2009.0 minimum by a factor of 2-4 reduction in the mass loss rate of η Car. Our more recent models account for the extended emission and absorption along the full wind-wind interaction shock front. For WR140, the computed X-ray light curves again match the RXTE observations quite well. But for η Car, a hot, post-periastron bubble leads to an emission level that does not match the extended X-ray minimum observed by RXTE. Initial results from incorporating radiative cooling and radiative forces via an anti-gravity approach into the SPH code are also discussed.

We present a novel theoretical tool to analyze the dynamical behaviour of a Be disk fed by non-constant decretion rates. It is mainly based on the computer code HDUST, a fully three-dimensional radiative transfer code that has been successfully applied to study several Be systems so far, and the SINGLEBE code that solves the 1D viscous diffusion problem. We have computed models of the temporal evolution of different types of Be star disks for different dynamical scenarios. By showing the behaviour of a large number of observables (interferometry, polarization, photometry and spectral line profiles), we show how it is possible to infer from observations some key dynamical parameters of the disk.

We present the results of magnetic field measurements for a sample of 23 young Herbig Ae/Be (HAEBEs) stars and 12 stars with debris disks. The spectropolarimetric data were obtained during four observing runs in 2003-2008 at the European Southern Observatory with the multi-mode instrument FORS 1 installed at the 8 m Kueyen telescope. Among the 23 HAEBEs studied, stellar magnetic fields of about 100-150G have been detected in 11 stars (i.e. ~50%). The presence of circumstellar polarization signatures formed in the stellar wind supports the assumption that the magnetic centrifuge is one of the main mechanisms of the wind acceleration. No field detection at a significance level of 3σ was achieved in stars with debris disks.

The Herbig Ae/Be stars are the high-mass counterparts of the T Tauri stars, and are therefore considered as the pre-main sequence progenitors of the A/B stars. These stars are still contracting towards the main sequence, and are surrounded by dust and gas, remnants of their parental molecular cloud. In order to understand the formation processes at high mass, as well as the magnetic and rotation properties of the MS A/B stars, it is fundamental to understand the structure of the circumstellar matter of the Herbig Ae/Be stars, as well as the interaction of these PMS stars with their close environment. In this talk I will review our current knowledge about the properties of the circumstellar environment of the Herbig Ae/Be stars as well as the possible physical processes at the origin of their observed activities.

We present preliminary results of the first near-infrared variability study of the Arches cluster, using adaptive optics data from NIRI/Gemini and NACO/VLT. The goal is to discover eclipsing binaries in this young (2.5 ± 0.5 Myr), dense, massive cluster for which we will determine accurate fundamental parameters with subsequent spectroscopy. Given that the Arches cluster contains more than 200 Wolf-Rayet and O-type stars, it provides a rare opportunity to determine parameters for some of the most massive stars in the Galaxy.

Mercury-manganese (HgMn) stars were considered to be non-magnetic, showing no evidence of surface spots. However, recent investigations revealed that some stars in this class possess an inhomogeneous distribution of chemical elements on their surfaces. According to our current understanding, the most probable mechanism of spot formation involves magnetic fields. Taking the advantage of a newly-built polarimeter attached to the HARPS spectrometer at the ESO 3.6m-telescope, we performed a high-precision spectropolarimetric survey of a large group of HgMn stars. The main purpose of this study was to find out how typical it is for HgMn stars to have weak magnetic fields. We report no magnetic field detection for any of the studied objects, with a typical precision of the longitudinal field measurements of 10 G and down to 1 Gauss for some of the stars. We conclude that HgMn stars lack large-scale magnetic fields typical of spotted magnetic Ap stars and probably lack any fields capable of creating and sustaining chemical spots. Our study confirms that alongside the magnetically altered atomic diffusion, there must be other structure formation mechanism operating in the atmospheres of late-B main sequence stars.

BI 108 is a luminous variable star in the Large Magellanic Cloud classified B1 II. The variability consists of two resonant periods (3:2), of which only one is orbital, however. We discuss possible mechanisms responsible for the second period and its resonant locking.

As part of a long term observational project, we are investigating the macroturbulent broadening in O and B supergiants (Sgs) and its possible connection with spectroscopic variability phenomena and stellar oscillations. We present the first results of our project, namely firm observational evidence for a strong correlation between the extra broadening and photospheric line-profile variations in a sample of 13 Sgs with spectral types ranging from O9.5 to B8.