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Algol is a triple system containing a 70 hr eclipsing binary (K IV and B8 V) in a 694 day orbit with an A V star. The X-ray emission from this system (Schnopper et al 1976) is thought to be associated with a corona surrounding the lobe filling and synchron ously rotating K IV star. This is based on the similarity of the X-ray spectrum and luminosity of this system to that of the RS CVn binaries which also contain K sub-giants with similar rotation periods and the fact that the luminosity of any coronae surrounding the B8 V and AV companion stars should not be enhanced by rapid rotation (Pallavicini et al 1980, White et al 1980). The Einstein SSS measurement showed the X-ray spectrum to be two component with 6 7 temperatures of 7. 10 K and 3. 10 K (White et al. 1980). As dis cussed by Swank et al. (1981), the problem in understanding stellar coronae in general is how to scale up the solar model to account for the enhanced luminosities. The close to 900 inclination and similar sizes for the Band K stars of 3. 6 and 3. 8 R0 respectively make Algol an ideal candidate for an X-ray eclipse measurement wherein the size of the X-ray emitting coronal structures can be directly measured. In this paper we report a continuous observation through the secondary eclipse of Algol using the EXOSAT Observatory. 2.

Since 1967. the most prominent events of a General Assembly of the International Astronomical Union are published in a separate volume. The Highlights of Astronomy (volume 9) report on the major scientific presentations made at the XXIst General Assembly July 23-August 1. 1991. Buenos Aires. Argentina. The present volume contains the texts of the three invited Di scourses and of the papers presented duri ng seven Joi nt Discussion Meetings and eight Joint Commission Meetings. The invited Discourses were arranged by the IAU Executive Committee and the Joint Discourses and Commission Meetings by the respective chairpersons. The overall responsability of the General Assembly was carried out by the IAU General Secretary (19B8-1991). Dr. D. McNally. I am indebted to the authors of the invited Di scourses. to the organi zers and editors of the scientific sessions for having provided me with all the material for publication in due time. I want to particularly acknowledge the case with which Dr. J.H. Hughes. who unfortunately died a few days ago from cancer. prepared and checked unti 1 the 1 ast few weeks the report on the new IAU Reference System. This report itself concludes long and difficult discussions among IAU members that he led in a wise and clear sighted manner.

A review of the current observational knowledge and understanding of the cosmic X-ray background.

Up-to-date accounts of recent and future advances in short-wavelength spectroscopy of laboratory and cosmic plasmas.

Clusters and superclusters of galaxies are the largest objects in the Universe. They have been the subject of intense observational studies at a variety of wavelengths, from radio to X-ray which has provoked much theoretical debate and advanced our understanding of the recent evolution of the large-scale structure of the Universe. The current status of the subject is reviewed in this volume by active researchers who lectured at a NATO Advanced Study Institute held in Cambridge, England in July 1991. Much of the material is presented in a pedagogical manner and will appeal to scientists, astronomers and graduate students interested in extragalactic astronomy.

The one-stop general book on the whole of X-ray astronomy.