We are developing a project aimed at studying the physical properties, origin and evolution of low-ionization structures in planetary nebulae. Within this project we have identified a number of pairs of highly collimated low-ionization jet-like features (Gonçalves et al. 2001). In spite of being very similar to real jets, they have the intriguing

We report on our continuing effort to identify extremely low-mass members of the cluster around the hot star sigma Orionis. Using the KeckI telescope in Hawaii and the Isaac Newton telescope in La Palma, we carried out a deep near-infrared and optical search in an area of 55 square arcminutes, which has yielded the discovery of an object with blue

We present NLTE abundances of CNO for a sample of four O9 stars in the Galaxy, together with new determinations of their stellar parameters, T_eff, log g, epsilon (He) and microturbulence. These new analyses take into account the effect of line-blocking in the spectral synthesis with our classical NLTE, plane-parallel and hydrostatic model

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An analysis of the behavior of the dust coma of the Comet C/1999 S4 (LINEAR) from visible and infrared images acquired shortly before and after the disruption of the comet's nucleus is presented. During the predisruption phase, the overall dust production increased by a factor of 11 in two steps; an initial outburst occurred between July 18 and 19