H1517+656 is an unusual source, even for a BL Lac object. It is one of the most luminous BL Lac objects known, with extreme emission properties at radio, optical, and X-ray frequencies. Furthermore, in our WFPC2 snapshot survey we discovered a series of peculiar arcs describing a 2.4" radius ring surrounding the source. This paper describes follow

We present accurate relative ages for a sample of 55 Galactic globular clusters. The ages have been obtained by measuring the difference between the horizontal branch and the turnoff in two internally photometrically homogeneous databases. The mutual consistency of the two data sets has been assessed by comparing the ages of 16 globular clusters in

We explore the formation and evolution of hydrogen-deficient post-AGB white dwarf stars by computing the complete evolution of an initially 2.7 Msun star from the ZAMS through the thermally pulsing and mass-loss phases, the born-again stage to the white dwarf stage. A time-dependent scheme for the simultaneous treatment of abundance changes caused

The destructive 1980 Campania-Lucania earthquake was a complex event, consisting of at least three subevents. While the main features of the first (0 s) and of the last (40 s) subevents are widely accepted, the second subevent (20 s) is still controversial. Fault geometries are here determined from levelling data, using a global minimization

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En este trabajo se realiza la implementación del método de extinción- distancia para determinar distancias a nebulosas planetarias, utilizando la clasificación espectral que realiza un sistema de redes neuronales artificiales. La aplicación del método mencionado exige la clasificación espectral de un numeroso grupo de estrellas de campo cercanas a