In an Omega 0 = 1 universe, within the classical Eulerian theory of gravitational instability, the redshift evolution of a peculiar velocity field in a region with arbitrary initial density contrast is derived, for the first time, in real space and third-order perturbation theory. A vector proportional to the gravitational acceleration can also be

We present a light curve of C/1995 O1 (Hale-Bopp) compiledfrom more than 3000 visual observations of the comet made by members of the The Astronomer Group world-wide. These observations cover the period from discovery through to the end of 1997. The light curve shows that the rate of brightening of the comet varied widely at different times, with

We have obtained I-, J-, H- and K-band light curves of the recurrent nova TCrB. We find that we can fit the J-band light curve only with a Roche lobe filling secondary star and a dark spot of radius 11 deg-26 deg (90 per cent confidence) centred at the inner Lagrangian point. We obtain limits to the binary inclination of 38 deg-46 deg (90 per cent

Line asymmetries and shifts are powerful tools for studying velocity fields in the stellar photospheres. Other effects, however, could also generate asymmetries, blurring the information of the velocity patterns. We have studied the shifts and asymmetries induced in the profiles of spectral lines by pressure effects. The best theoretical and

Even the most sensitive cosmic microwave background anisotropy experiments have signal-to-noise ratios

We present the first observational evidence for a collimated jet in a short-period cataclysmic variable system: the recurrent nova T Pyxidis. Optical spectra show bipolar components of H alpha with velocities ~1400 km s-1, very similar to those observed in the supersoft X-ray sources and in SS 433. We argue that a key ingredient of the formation of