A full understanding of the properties of substellar objects is one of the major challenges facing astrophysics. Since their discovery in 1995, we have discovered hundreds of brown dwarfs and extrasolar planets. While these discoveries have enabled important comparisons with theory, observational progress has been much more rapid than the

Although the stellar and substellar populations have been studied in various young and old open clusters, additional studies in clusters in the the age range from 5 to 100 Myr is crucial (e.g. to give more constrains on initial mass function variation with improved statistics). Among the open cluster candidates from recent studies, two clusters are

The determination of atmospheric parameters is the first and most fundamental step in the analysis of a stellar spectrum. Current and forthcoming surveys involve samples of up to several million stars, and therefore fully automated approaches are required to handle not just data reduction but also the analysis, and in particular the determination

We present an analysis of Chandra High Energy Transmission Grating (HETG) spectra of the corona of the critical intermediate-activity, quadruple stellar system ξ UMa. Using the maximum A/B subsystem separation in 2001, we attempted to resolve for the first time in X-rays the two components using the HETG Medium Energy Grating (MEG). We found the Aa

Frequency differences between high-order p-modes of two stellar models, both with the mass and luminosity of the sun, are compared with results of asymptotic theory. The computed relative differences are closely approximated by averages of the relative sound-speed differences, weighted by the local sound travel-time, over the regions where the

We analyze the stellar distribution in the Galactic disc via NIR color-magnitude diagrams (CMDs). Extinction measurements in the K band are also extracted from the diagrams both for supporting stellar counts predictions of the Besançon and "SKY" models and for comparing with a three dimensional model of the Galactic extinction. The method is based