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The BL Lacertae object S5 1803+784 has been monitored with very long baseline interferometry at λ= 3.6 cm in 43 epochs between 1986.21 and 1993.95. The motivation of this work is to obtain statistically meaningful data with which to study the short‐term structural variability in the source on monthly time‐scales. We present a detailed analysis of the structural evolution seen in the milliarcsecond‐scale jet by way of model‐fitting results. Within the simplest identification scenario, the jet can be described as a bent chain of seven jet components within 0.2‐ and 3‐mas separation from the core. New components seem to emerge almost regularly (every 2 yr) from the core. Three jet components approach the brightest and so‐called ‘stationary’ component (at ∼1.4 mas at 8.4 GHz) with apparent superluminal motion of 8–11c. In this paper, we show that the ‘stationary’ component (under the assumption that this is the brightest jet component in each epoch) oscillates, and we discuss several possible explanations. We show that a reconfinement shock can easily reproduce the observed most recent oscillation of the ‘stationary’ component. We present evidence that the average jet ridge line is significantly curved and discuss the possibility that the jet components follow a helically bent path.

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VLBI observations of the extremely y-bright blazar PKS 0528+134 at 8, 22, 43, and 86 GHz reveal a strongly bent one-sided-core jet structure with at least three moving and two apparently stationary jet components. At the highest observing frequencies the brightest and most compact jet component (the VLBI core) is unresolved with an upper limit to its size of '50 microarcsec corresponding to "0.2 parsec [Ho = 100 km s'l Mpc-1 (megaparsec-1), qo = 0.5, where Ho is Hubble constant and qo is the deceleration parameter]. Two 86-GHz VLBI observations performed in 1993.3 and 1994.0 reveal a new jet component emerging with superluminal speed from the core. Linear back-extrapolation of its motion yields strong evidence that the ejection of this component is related to an outburst in the millimeter regime and a preceding intense flare of the y-flux density observed in early 1993. This and the radio/optical "light curves" and VLBI data for two other sources (S5 0836+710 and 3C 454.3) suggest that the observed y-radiation might be Dopplerboosted and perhaps is closely related to the physical processes acting near the "base" of the highly relativistic jets observed in quasars.The high-redshift blazar PKS 0528+134 (redshift z = 2.07), recently attracted special attention due to its extreme brightness and pronounced variability in the y-ray regime (1, 2). Previous very-long-baseline radio interferometry (VLBI) observations at 8 GHz and 22 GHz revealed a core jet morphology, extending several milli-arcseconds (mas) to the north (3) [1 mas corresponds to =4 parsecs (pc), assuming a cosmological origin of the redshift of the source, and Ho = 100 km s-l Mpc-1, qo = 0.5 where Ho is the Hubble constant and qo is the deceleration parameter] with at least one superluminally moving jet component (4). It was also suggested that at least part of the radio variability observed at centimeter wavelengths might be due to refractive interstellar scattering (4).With their high angular resolution, VLBI observations at millimeter wavelengths allow detection of new jet components shortly after their "birth" in the immediate vicinity of the hypothetical central black hole, tracing their motion from several tens to several hundreds of micro-arcseconds (,uas) core separations (5, 6). These central regions of VLBI jets are synchrotron self-absorbed at longer wavelengths and therefore cannot directly be imaged at centimeter wavelengths. RESULTSIn May 1992, the blazar 0528+134 was observed at 43 GHz within a global VLBI observing campaign with eight stationsThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.participating. Despite a somewhat limited ,v-coverage (6), the source could be imaged for the first time at 43 GHz, and several distinct jet components were found. These components are also seen in a 22-GHz VLBI map obtained 6 months later (4). In Fig. 1 both images (43 GHz and 22 GHz)...

We propose a small interferometer on a minisatellite, named DIVA (Deutsches Interferometer fiir Vielkanalphotometrie und Astrometrie), to perform astrometric and photometric observations of at least one million stars. It would exceed the performance of Hipparcos in all important parameters. With a possible launch in 2002 DIVA would provide for all stars brighter than V = 10m5 -parallaxes better than 0.8 milliarcseconds (mas) -proper motions better than 2 mas/y -broad-band photometry with a typical accuracy of 0.002 mag -multi-channel intermediate-band photometry with a typical accuracy of 0.01 to 0.02 mag For the 100 000 stars observed by Hipparcos, proper motions with a typical accuracy of 0.1 to 0.2 mas/y and parallaxes of 0.3 to 0.5 mas can be obtained in combination with the Hipparcos results. The scientific potential of this mission and a possible layout of its optical system are described in this paper.

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From June 15-28, 1991 the Compton Gamma-Ray Observatory (CGRO) observed the radio-loud quasar 3C 273. All four CGRO instruments detected radiation from this quasar in their relevant energy range (from 20 keV to 5 GeV). Simultaneous and quasi-simultaneous observations by instruments sensitive at other wavelengths have also been obtained. The data from all these observations, spanning the frequency range from ,,~ 109 Hz- ,-~ 1026 Hz, were collected and analysed. Details of the observations and an overall energy-density spectrum are presented. This spectrum shows two maxima of nearly equal strength. One is in the UV, while the other one is found at low-energy "r-rays. The implications of these simultaneous observations on some theoretical models will be discussed.