Aims.We searched for photometric variability in a sample of hot, hydrogen-deficient planetary nebula nuclei (PNNi) with "PG 1159" or "O VI" spectral type, most of them embedded in a bipolar or elliptical planetary nebula envelope (PNe). These characteristics may indicate the presence of a hidden close companion and an evolution affected by episodes

A consistent analysis, using non-LTE codes with sphericity and mass loss, of different ionization species in O star spectra observed from the far-UV to UV allowed us to constrain the long-time uncertain ionization equilibrium in the stellar winds, to quantify the role of soft X-rays from shocks and to reconcile otherwise discrepant UV line

Environmental differences in the stellar populations of early-type galaxies are explored using principal component analysis (PCA), focusing on differences between elliptical galaxies in Hickson Compact Groups (HCGs) and in the field. The method is model-independent and purely relies on variations between the observed spectra. The projections (PC1

The formation mechanism of brown dwarfs (BDs) and its relation to star and planet formation remains an open issue. A careful determination of the substellar mass function in open clusters at different ages and environments is one of the primary methods for addressing this. We have performed a deep, wide area, multiband photometric survey from which

We describe the ZEUS phasing camera for future extremely large telescopes (ELTs) based on the Zernike phase contrast method. A prototype instrument is under construction for implementation in the Active Phasing Experiment (APE), a VLT test bed scheduled for operation in 2007. The paper describes theoretical aspects of the method and its

Observations of M31 were obtained using the 0.8-m telescope at Wendelstein, Germany and the 1.2-m telescope at Calar Alto, Spain, over three observing seasons, from 2000 to 2003. Two broad-band filters, R and I close to Kron-Cousins system, were employed. (3 data files).