This thesis is made of three main parts. In the first one a sample of 55 galaxies with an active nucleus (Seyfert 1, Seyfert2 and LINERs) is analysed; these were observed with the 4.2m WHT and 1m JKT in CCD narrow band H-alpha +[NII] and [OIII] to map the distribution of HII regions and the morphology of the circumnuclear extended emission

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We present LTE and NLTE results on the formation of LiI lines (λ6103, λ6708, and λ8126) in the atmospheres of solar metallicity dwarfs with effective temperatures in the range 5500K to 2000K. NLTE effects are governed by overionization of Li and by the interlocking effects of energy levels. For stars with T_eff_>=4000K, we confirm previous findings

We report the first spectroscopic detection of the secondary star in the X-ray transient QZ Vul (=GS 2000+25). Cross-correlation of individual spectra against K-type templates yields clear velocity modulations with the 8.3-h orbital period. From the amplitude of the radial velocity curve (520+/-16 km s^-1) we derive a mass function f(M)=5.02+/-0.46

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