Observations of the gravitational lens system 0957+561A, B in the R band are presented in this Letter. The observations, taken with the 82 cm IAC-80 telescope, at Teide Observatory, Spain, were made from the beginning of 1996 February to June, as part of an on-going lens monitoring program to obtain the time delay between the two components of the

We present a set of face-on models of the radiative transfer of stellar light in exponential discs, taking into account absorption and multiple scattering (diffusion) by dust. Attention has been directed towards the effects on the observable photometric properties of different distributions of stars and dust perpendicular to the galactic plane. A

We present the results of an intensive ultraviolet monitoring campaign on the Seyfert I galaxy NGC 4151, as part of an effort to study its short-timescale variability over a broad range in wavelength. The nucleus of NGC 4151 was observed continuously With the International Ultraviolet Explorer for 9.3 days, yielding a pair of LWP and SWP spectra

A three-wavelength sun-photometer was designed and built. The attenuation of direct solar light, at wavelengths of 0.38 micrometers , 0.50 micrometers and 1.00 micrometers were measured. Silicon photodiodes and interference filters of 10 nm FWHM were used. Aerosols were characterized by angstrom's turbidity formula in terms of coefficients (alpha)

We have obtained broad-band optical spectra of a sample of 15 low-mass X-ray binaries (LMXBs) in the range 4000-9000A and fitted them with combinations of standard accretion disc models plus secondary star templates. For the sample of LMXBs in which we determine the power-law indices of the accretion disc flux distribution, we can explain the

We investigate the influence of scattering processes in the continuum on the formation of lines in static, one dimensional atmosphere. It is shown that the Schuster mechanism of emission line formation can work under certain physical conditions. We derive formulae for the calculation of the line profiles in atmospheres with an ad hoc temperature