We present intermediate dispersion optical spectroscopy for seven very low-mass stars in the α Per open cluster with spectral types between M3 and M6, including the brown dwarf candidate of Rebolo et al. (1992). Our radial velocity measurements are found to be generally consistent with the mean cluster velocity to within the measurement errors. Hα

A new, very fast approximate method is presented to determine mass-loss rates of O-stars from Halpha_ line profiles. The method uses H and HeII departure coefficients from unified model atmospheres parametrized in a simple way as function of wind velocity together with photospheric NLTE line profiles as the inner boundary condition for a

Modelling of asymmetries in spectral lines with different formation heights (K I 7699, Fe I 5576 and Fe I 5635A), observed in the quiet Sun at the disk center, has been performed with the aim of reproducing the absolute line bisectors obtained from low resolution observations. The influence of a number of physical and observational factors on the

Comet Hale-Bopp is, without doubt, one of the most important objects for cometary science which has ever been observed. The light curve is well-observed from a heliocentric distance 7.2 AU and some information is available at distances as great as 17 AU. This allows the photometric evolution of the comet to be studied as different volatiles

A comprehensive study of the planetary nebula K4-55 is presented. In addition to CCD narrowband images, spatially resolved low-dispersion and high-resolution spectra were obtained. This has allowed the different morphological features in the images to be connected with the kinematics and with the physical information, leading to the establishment

The first X-ray observation of the Seyfert 2 galaxy NGC 5252 is reported. ASCA Astro-D collected approximately 4000 photons per detector, enough to perform an accurate spectral analysis of this source. The luminosity of NGC 5252 is LX(0.7-10 keV) ≌ 2.6 × 1043 ergs s-1, typical of a Seyfert 1 galaxy. A simple description of the spectrum with a