The observational picture of the enigmatic O-type spectrum variable HD 191612 has been sharpened substantially. A symmetrical, low-amplitude light curve with a period near 540 days has recently been reported from Hipparcos photometry. This period satisfies all of the spectroscopy since at least 1982, including extensive new observations during 2003

We present a catalogue of previously unpublished optical and infrared photometry for a sample of 162 emission-line objects and shell stars visible from the southern hemisphere. The data were obtained between 1978 and 1997 in the Walraven (WULBV), Johnson/Cousins (UBV(RI)c) and ESO and SAAO near-infrared (JHKLM) photometric systems. Most of the

It is essential to properly calibrate the polarimetric properties of telescopes, if one wants to take advantage of the capabilities of high precision spectro-polarimeters. We have constructed a model for the German Vacuum Tower Telescope (VTT) that describes its time-dependent polarization properties. Since the coelostat of the telescope changes

The population of high-energy and very high-energy γ-ray sources, detected with EGRET and the new generation of ground-based Cherenkov telescopes, forms a reduced but physically important sample. Most of these sources are extragalactic (e.g. blazars), while among the galactic ones there are pulsars and supernova remnants. The microquasar LS 5039

Observations of binary stars containing an accreting black hole or neutron star often show x-ray emission extending to high energies (>10 kilo­–electron volts), which is ascribed to an accretion disk corona of energetic particles akin to those seen in the solar corona. Despite their ubiquity, the physical conditions in accretion disk coronae remain

A technique for obtaining an ideal two-mirror coude system with no instrumental polarization is described. A half-wave plate positioned between the two mirrors with the proper orientation produces the desired effect. The level of spurious polarization is limited by the characteristics of the retarder and the similarity of the mirrors. The telescope