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Aims. We try to identify ultra-cool dwarfs from the seventh Data Release of the Sloan Digital Sky Survey (SDSS DR7) with SDSS i-zand r-zcolors. We also obtain proper motion data from SDSS, 2MASS, and UKIDSS and improve spectral typing from SDSS and 2MASS photometric colors.

We present a near-infrared spectroscopic survey of a large area centered on the Cygnus OB2 association aimed at constraining its massive star contents. Our goal is to establish a nearly complete list of O-type members of the association, both to examine recent claims based on starcounts that suggest a richer content than previously thought, and to provide a suitable database for further studies of the entire high-mass end of one of the richest associations of the Galaxy. The target selection is based on the JHKphotometry published in the 2MASS all-sky survey. We identify 46 new early-type candidates, most of them expected to be O-type stars, plus 16 new stars with emission in Brγand often in other lines as well, characteristic of evolved massive stars undergoing intense mass loss. We also present spectra of three luminous stars with CO overtone emission, one of them having also intense H2emission and being associated with compact nebulosity. By considering our findings, those of other authors, and plausible completeness corrections, we estimate the number of O-type stars or stars having evolved from a O-type progenitor to be 90–100, slightly below, but compatible with, most recent starcounts estimates by Knödlseder (2000, A&A, 360, 539). These results support the notion that Cygnus OB2 may be considered as a young globular cluster. The lists of new members that we provide, in particular those with emission lines, should be a useful resource for future investigations of Cygnus OB2 itself, as well as of very massive stellar evolution by providing a nearby, abundant sample of stars sharing a common environment.

Context.Chemical abundances of light elements such as beryllium in planet-host stars allow us to study the planet formation scenarios and/or investigate possible surface pollution processes.Aims.We present here an extension of previous beryllium abundance studies. The complete sample consists of 70 stars that host planets and 30 stars without known planetary companions. The aim of this paper is to further assess the trends found in previous studies with fewer objects. This will provide more information on the processes of depletion and mixing of light elements in the interior of late-type stars, and will provide possible explanations for the abundance differences between stars that host planets and “single” stars.Methods.Using high-resolution UVES spectra, we measure beryllium abundances of 26 stars that host planets and one “single” star mainly using the λ3131.065 Å Be ii line, by fitting synthetic spectra to the observational data. We also compile beryllium abundance measurements of 44 stars hosting planets and 29 “single” stars from the literature, resulting in a final sample of 100 objects.Results.We confirm that the beryllium content is roughly the same in stars hosting planets and in “single” stars at temperatures Teff≳ 5700 K. The sample is still small for Teff≲ 5500 K, but it seems that the scatter in Be abundances of dwarf stars is slightly higher at these cooler temperatures. Conclusions.We search for distinctive characteristics of planet hosts through correlations of Be abundance versus Li abundance, age, metallicity, and oxygen abundance. These could provide some insight into the formation and evolution of planetary systems, but we did not find any clear correlation.