This thesis presents observational kinematic results for a significant sample of disk galaxies. Spectra of both the gaseous and stellar components are available for almost the whole sample. The first aim was to make a comparative study of the gas and the stellar kinematics to see whether there was any correlation between the kinematic behavior and

IAUC 7273 available at Central Bureau for Astronomical Telegrams.

We present near-infrared photometry (J band) and low-resolution optical spectroscopy (600-1000 nm) for one of the faintest substellar member candidates in the young σ Orionis cluster, S Ori 47 (I=20.53; Béjar, Zapatero Osorio, & Rebolo). Its very red (I-J) = 3.3+/-0.1 color and its optical spectrum allow us to classify S Ori 47 as an L1.5-type

We present very accurate orbital parameters and mass measurements (2.4% accuracy) for the well known very low mass triple system Gl 866. We obtain first orbital elements for the short-period orbit and greatly improve the long period orbit. All three stars have masses close to 0.1 M_sun, and the system thus provides the strongest constraints to date

We present results of an extensive study of the X-ray spectral properties of sources detected in the RIXOS survey, which is a large, nearly complete sample of objects detected serendipitously in ROSAT PSPC fields down to a flux limit of 3x10^-14ergcm^-2s^-1 (0.5-2keV). We show that for X-ray surveys containing sources with low count rate, such as

The stratification with geometrical height of the velocity and temperature 5 minute oscillation through the solar photosphere is obtained after the inversion of a time series of KI 7699 Å line spectra taken at disk center.