We have observed the high excitation planetary nebulae NGC 7027, NGC 6543 and the low excitation planetaries Hu 2-1 and Cn 3-1 in the optical & near IR with the aim to contribute accurate measurements of their gaseous iron abundances. Use was made of multi-level atomic models of iron ions to interpret the observed forbidden lines of various iron

We propose spectroscopic observations of two new gravitational lens candidates, 0502+675 and 1440+122, discovered in our HST R-band snapshot survey of ~100 BL Lac objects. Each contains a double point source with separation of only 0.3 arcsec. In each case, a candidate for the lensing galaxy is also found. For 0502+675, a subsequent NICMOS H-band

En esta Tesis se analiza la estructura de una mancha solar tipica, redonda, aislada y situada cerca del centro del disco solar, para 25 capas en profundidad óptica. Los datos fueron obtenidos con el "Advanced Stokes Polarimeter", uno de los mejores instrumentos disponibles, y la estratificación de los diferentes parámetros a lo largo de la

We have obtained high-resolution echelle spectroscopy of the recurrent nova T CrB. We find that the surface lithium abundance in T CrB is significantly enhanced compared with the case of field M giants, where it is not detectable. We offer possible explanations for this in terms of either a delay in the onset of convection in the giant star

En la tesis se presenta un método para la inversión de los perfiles de Stokes de lineas espectrales para lasque la aproximación de Equilibrio Termodinámico Local no es adecuada. Este método es aplicado a observaciones espectro-polarimétricas realizadas con vistas al estudio de la dinámica y el magnetismo de la cromosfera sobre laumbra de las

We discuss results on the oxygen abundance in a sample of 23 metal-poor (-3.0≤ [Fe/H] ≤ -0.3) unevolved stars and one giant. High resolutionspectroscopy of OH lines in the near UV allowed us to trace the early evolution of oxygenversus metallicity. Contrary to previous expectations, we find that oxygen abundances derived from these low excitation