Light curves of quasars give information on time scales (hence limits on size of principal luminous source), and on phase of variation (hence evidence bearing on the distance of surrounding regions or shells).

In 1979 we started a monitoring program of Seyfert galaxies, using various telescopes at ESO (La Silla, Chile). At present this program is still continuing, but here we discuss only the photometric data obtained in the period 1979–1982.

Using the VLA at 6 cm, we have detected non-periodic variations in the powerful BL Lacertae object OJ287 on timescales between minutes and hours at levels of the order of one percent. Brightness temperatures inferred from causality arguments range from 1016 to 1020 K. No periodic component was found to a limit of 0.1%.

Shortly after their discovery, it was realized that most quasars are not found in radio source catalogues, and a number of searches were initiated to try to detect weak radio emission from “optically selected quasars.”

We describe models which try to account for the incidence of quasar radio emission in terms of two populations. One is effectively radio quiet, with radio powers P2.7 ≤ 1022 WHz−1 sr−1 characteristic of type I Seyferts. These are presumed to lie in spiral host galaxies. The other population is the classical radio-loud quasars with P2.7 ≥ 1024 WHz−1 sr−1. Apparent correlation of radio and optical powers can arise without any real effect being present because the two populations in general have separately-evolving luminosity functions.

It is still uncertain whether optical and radio emission mechanisms in AGNs are coupled or not, and how? New observational data give more light on this subject.

Using a large, unbiased sample of 374 optically selected QSOs, of which 54 are radio detected, we have computed for different redshift ranges the probability distribution function φ (R) of R defined as the ratio of monochromatic luminosities at 15 GHz and 2500 Å in the QSO's rest frame. A significant variation of φ (R) with redshift is noticed. At small redshifts (z < 0.5), the distinctive feature of φ (R) is a peak near R∼102, arising due to the QSOs having steep radio spectrum(αr<−0.5).

In recent years a number of very steep spectrum, compact radio sources have been discovered (e.g. Cotton 1983, Cotton and Owen 1985, Ulvestad 1985) which have no optical counterpart to the limit of the Palomar Sky Survey. VLBI observations of a number of these have confirmed the very compact (<10 mas) nature of several of these sources. Analysis of the available data in terms of the standard synchrotron modal suggest that they contain very weak magnetic fields, large particle densities and may emit detectable infrared and optical emission by inverse Campton scattering in the compact radio source (Cotton 1983). This paper will report on an analysis including new VLBI observations, infrared and optical imaging at KPNO and low frequency radio observations at CLRO of a number of these objects.

We have obtained direct images and off-nuclear spectra for five QSOs having a wide range of radio, optical and X-ray luminosity. Four objects show absorption features identified with stars in a host galaxy, and off-nuclear changes in emission line wavelengths. All objects show off-nuclear changes in continuum coulour, and in emission line intensities and ratios. The radio loud objects have more luminous galaxies, strong and extended [OII] and [OIII], and UV-bright nuclei. They tend to have luminous nuclei, high nuclear to galaxy luminosity ratio, and blue host galaxies. This paper is a brief summary of results that will appear in more detail elsewhere.

It is believed that the QSO phenomenon is a form of violent activity in the nuclei of galaxies. This view has found support after the recent detection of nebulosity around a large number of quasars. The nature of the fuzz is not yet clear, but there is some evidence that it is normallly a spiral galaxy hosting a quasar (e.g. Hutchings et al. 1984). However, the experience with radio galaxies suggests that the host of a radio loud quasar ought to be an elliptical galaxy. We have considered the appearance of elliptical galaxies of various redshifts with a view towards determining the nature of ellipticals which could act as hosts for radio quasars at large redshifts. For spirals we have considered the data given by Boroson (1981) and calculated the appearance of different morphological types at large redshifts.

We present here the first results of a program to obtain CCD images of a number of southern extragalactic radio objects using the 3.9-m Anglo-Australian telescope.

A long term project was started at the Max-Planck-Institut fur Astronomie, Heidelberg, to study a complete sample of 46 northern and southern BL Lac objects with flux densities exceeding 1 Jy at 5 GHz using optical spectroscopy, optical polarimetry, and direct deep CCD imaging.

We present preliminary results of a CCD survey of nearby (z<0.1) northern galaxies which have well defined radio jets. Our main objectives are to: 1) detect - or put constraints to - the presence of optical jets in these galaxies, 2) obtain high dynamic range maps of known optical jets, and 3) make a detailed comparison of our results with those available from the radio maps.

The region surrounding the nucleus of 3C 120 has been recorded on electronographic plates, in U, B, V and V' colours. It has been possible to substract the contributions of the bright nucleus and the galaxy. Faint condensations appear. Their colours show that they are probably not emitting synchrotron radiation. The brightest condensation A is located 4” West of the nucleus. It does not coïncide exactly with the knot of the radio jet and is very slightly displaced toward the North. It is probably of the same nature as Minkowski's Object: burst of star formation triggered by the collision of the jet with relatively dense extranuclear gas.

Recent results concerning the compact radio structure of quasars are reviewed. Emphasis is placed on VLBI results from statistical studies, from detailed mapping and from multi-epoch monitoring of variable sources.

We present milliarcsecond-resolution 5 GHz polarization maps of several active galactic nuclei: one epoch each for the quasar 3C345, the galaxy 3C120, and the BL Lacertae object 0735+178, and two epochs for the BL Lacertae object OJ287.

The flux variability of extragalactic radio sources at decimetric wavelengths (Low Frequency Variability LFV) is mostly associated with the nuclei of compact radio sources. But is a not yet well understood phenomenon. The main question still is: where does this phenomenon take place?

The quasar 3C 345 is recognized as a strong, variable, superluminal radio source, an optically violent variable, and a weak X-ray source. We present results of VLBI monitoring between 1979 and 1984 at 2.3, 5.0, 10.7, and 22.2 GHz using antennas of the U.S. and European VLBI Networks. These results are interpreted in terms of a relativistic jet model (Biretta 1985; Biretta et al. 1986).

The quasar 3C273 lies at a declination of +2°. Maps made with northern hemisphere VLBI Networks clearly show superluminal motion, but the north-south resolution is poor and details cannot be seen. We have now made two observations at 10.7 GHz with the Itapetinga Radio Observatory in Brazil, and one at 5.0 GHz using the Hartebeesthoek Observatory in South Africa along with the European and U.S. VLBI Networks (Biretta et al. 1985).