The probabilistic nature of the IMF in stellar systems implies that clusters of the same mass and age do not present the same unique values of their observed parameters. Instead they follow a distribution. We address the study of such distributions in terms of their confidence limits that can be obtained by evolutionary synthesis models. These

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Recent determinatios of baryonic density using the angular power spectrum of the Cosmic Microwave Background are very close to the classical estimate from Big Bang Nucleosynthesis. This reinforces the case for dark baryons in the Universe and for a large component of exotic cold dark matter. Present-day baryons can be hidden in substellar objects

En esta tesis se aborda el estudio de los espejismos gravitatorios QSO 0957+561 y QSO 2237+0305. Para ello se ha realizado un seguimiento fotométrico de 5 años del sistema QSO 0957+561 en el telescopio IAC-80 del Observatorio de Teide. Utilizando una nueva técnica de reducción fotométrica y un nuevo método de obtención del retraso temporal, se ha

We report the discovery of an unusual low-ionization broad absorption line quasar at z=1.776 which exhibits absorption lines from many excited states of Fe II. This member of the rare class of ``FeLoBAL'' quasars was serendipitously found in a mid-infrared (MIR) survey of distant clusters carried out with the ISOCAM camera. ISO J005645.1-273816 has

Here we make a new study of the behaviour of the NaI/KI column density ratio in the interstellar medium, using a sample of new observations of 28 stars obtained at the Nordic Optical Telescope (NOT) in 1996 and 1997, and previously published observations (obtained by some of the authors) of 21 stars. The sightlines cover a range of distances and