We present intermediate dispersion (0.7-2.2 A/pixel) optical spectroscopic observations aimed at applying the 'lithium test' to a sample of 10 brown dwarf candidates located in the general field, two in young open clusters, and two in close binaries. We find evidence for strong Li depletion in all of them, and thus infer lower mass limits of 0.065

ESTA MEMORIA PRESENTA UN TRABAJO DE 4 AÑOS DE CONTRIBUCION, CON EXITO, A LA INTRODUCCION DE LOS CCDS, COMO DETECTORES FOTOMETRICOS PRECISOS, EN EL MUNDO DE LA ASTROSISMOLOGIA. SE EXPONEN Y DISCUTEN LOS DIFERENTES METODOS DE REDUCCION DE DATOS OBTENIDOS CON FOTOMETRIA DIFERENCIAL RAPIDA, COMPARANDO VARIOS PROGRAMAS DE REDUCCION Y PROPONIENDO ALGUNOS

In this work we analyze time-resolved spectra of SS Cygni in quiescence. The geometrical and orbital fundamental parameters of the system are determined. When in the data the contribution of the secondary star is removed some weak emission lines and a Fe I absorption line become clearly visible. We detect strong emission from the secondary star due

We make a quantitative comparison between spectral vs filter measurement and analysis techniques for extraction of solar vector magnetic fields from polarimetric data using as a basis the accurately calibrated, high angular resolution. Stokes profile data from the Advanced Stokes Polarimeter. It is shown that filter-based measurements deliver

Radial velocity observations performed using a resonant scattering instrument from 1980 to 1993 at the Observatorio del Teide have been used to calculate the monthly power spectra of very low degree solar oscillations. Cross-correlations among the spectra allow to determine the p-mode frequency shifts along the observed period. These shifts show a

We report the results of a CCD investigation of stars in a selected region near the center of the intermediate age galactic cluster NGC 7789. A CM diagram is presented for a sample of 1122 stars, which should be complete down to V=20. On the ground of recent theoretical evolutionary models, simulations of both the colour-magnitude diagram and the