The 3CRR sample of radio galaxies and quasars has been cross-correlated with the ROSAT PSPC All-Sky Survey. About 25 per cent of the objects have been detected in the 0.2-2.4 keV range, the large majority being powerful radio and X-ray emitters with a range of X-ray luminosities between 10^44 and 10^46 erg s^-1. The 0.2-2.4 keV spectrum of the

(Hα+[NII])/[OIII] ratio images have been used to search for low-ionization small-scale structures in a sample of 258 planetary nebulae. Radially symmetrical knots or jet-like structures, which are enhanced in the ratio images compared to the neighbouring regions of the nebulae, have been identified in 23 objects. Some of these features are barely

This paper presents first complete uvby light curves of the late-type detached eclipsing binary ZZ UMa (G0V + G8V, P=2.2993d). This binary system has been observed during eight campaigns at the Calar Alto Observatory (Almeria, Spain) and at the Sierra Nevada Observatory (Granada, Spain). 294 points distributed over the binary period and covering

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We have calibrated the effective temperatures of the low main sequence stars ranging spectral types from F0 to K5 versus [Fe/H] and colours (B-V), (R-I), (V-R), (V-I), (V-K), (J-H), (J-K) and ubvy-β, using a large sample of dwarfs and subdwarfs. The effective temperatures, scaled to direct T_eff_ determinations via reliable angular diameter

We present a procedure for obtaining the cluster densities in terms of the probability, P_n_(r), for a randomly placed sphere of radius r to contain n galaxies and the cluster density profiles. We then discuss a method for computing P_n_(r) and use it to obtain the density of Abell clusters in cold dark matter models with different biasing.