During the past three years there has been increased activity in many fields where photometric methods are employed; more telescopes have been commissioned or adapted for photometric measurements and new magnitude systems have been proposed and tried, but much of the observing is now of a routine nature. Most of this continues to be done in the wide-band UBV system, but more measurements are now being made in the red and infra-red.

More interest is being shown in narrower band-widths that emphasize particular features of a stellar spectrum, because these offer better prospects of separating the effects of temperature, luminosity, interstellar reddening, composition and duplicity. Attention has been given to finding the most useful bands and practical ways of measuring them, both photoelectrically and photographically.

On constate que dans l’ensemble les observations classiques se font en quantité égale à celle de 1964 à 1967. Les observations photographiques sont plus nombreuses.

De nouvelles méthodes de mesure sont essayées et détectent des couples inaccessibles aux méthodes classiques. Nous citerons par exemple les observations photoélectriques d’occultation de binaires par la Lune (Rakos), les mesures d’intensité interférométrique par comptage de photons (Twiss). Dans les deux cas des séparations très inférieures à 0”01 sont mesurées. Ces travaux semblent ouvrir la voie à une génération nouvelle de moyens d’observations de binaires.

This Report in arrangement of the material follows closely that written by Dr. Herbig for the last IAU General Assembly. It consists of four parts, each written by a different author. The first deals with variable stars in general, and was prepared by the President of Commission 27, with the exception of a section on the Theory of Variable Stars, very kindly written at his request by Prof. Kippenhahn. Appendix I on the Spectra of Variable Stars was prepared by Dr M. W. Feast, Chairman of that Committee of Commission 27. Appendix II covers the Variable Stars in Clusters, written by Dr Helen Sawyer-Hogg, Chairman of that Committee. Appendix III is a Report of the Working Group on Flare Stars, prepared by Drs Andrews, Chugainov, Gershberg and Oskanjan. No report has been received from the Working Group for Nomenclature for Magellanic Cloud Variables. About supernovae see the report of the Committee of Commission 28 for Research on Supernovae. On pulsars see the Reports of Commissions 25 and 40. Our thanks are due to all members of the Commission and to other astronomers occupied in variable star research, who have assisted in collecting the material for this report. Great help was a general report on flare stars by the competent Working Group.

The field covered by this report overlaps particularly with the work of Commissions 29 and 45 whose reports should be consulted. I have been greatly helped by reports on Soviet work by Professor Kukarkin and Dr Boyarchuk and a bibliography of European literature by Bloch.

The Sonneberg programme for determining spectral types of variables has been described by Götz and Wenzel (AN, 290, 99) and several lists have been published in MVS. Bakos is studying high dispersion spectra of a number of variables that are members of visual binary systems.

Stephenson and Terrill (ApJ, 147, 148) and Houk (IB, 228) have given spectral types for a considerable number of long period variables. Terrill (AJ, 74, 413) has made an extensive study of the variation of spectrum with phase for 51 red variables. Deutsch, Keenan and Garrison have continued their extensive programme of spectral classification of Mira variables. Keenan and Dessy (ApJ, 146, 583) suggest that the relatively early spectral type of the very long period Miras (R Cen, R Nor etc.) showing double maxima indicates that the fundamental parameter is the half-period (between successive maxima). A similar conclusion (ApJ, 93, 380; MN, 125, 367) was reached earlier by others on the basis of the correlation with period of the difference between absorption and emission velocities. Feast has used K subgiant companion of U Men to derive an absolute magnitude of — 0m.5 for the Mira star. The radial velocity of the companion agrees with the absorption velocity of the variable near maximum (Obs.).

Because of space restrictions, this report on the variable stars in clusters differs in form from those in earlier IAU reports, and is more condensed. Not all of the papers which have appeared since the last IAU General Assembly can be mentioned.

A definite trend is apparent in this field. A few years ago the aim of many programs was to detect new variables or to determine the periods of those known. While this is still being carried on, especially for the clusters in the far south, the emphasis has subtly changed to an avid pursuit of information about individual variables. For many variables, much more is known now than just the periodicity of their magnitude variations in photographic light. The number of observers studying these stars and the number of telescopes used in the program have greatly increased.

The Working Group was organized in order to promote and develop the cooperative studies of flare stars. Up to the present time our observation programmes were restricted to the UV Ceti-type stars only. Organizational activities followed the aims:

• (1) To extend the existing network of photoelectric observers in order to provide 24-hour coverage of a star during selected sessions lasting usually 14 days.

• (2) To collect data using standardized techniques in order to study frequency of stellar flares and secondary variations.

• (3) To provide optical coverage for simultaneous radio, spectroscopic and polarimetric observations.

The volume of work published on extragalactic astronomy in the period August 1966 – July 1969 has been large. Limitations of space have precluded the writing of an all-inclusive report; in all sections some selection has had to be made. References are given, whenever possible, by the reference numbers of the Bulletin Signalétique of the CNRS of France, with the omission of the Section number: “2” for vols. 27 to 29 and “120” for vol. 30. A reference number preceded by J is taken from the Astronomischer Jahresbericht. When no reference numbers were available, abbreviated journal references have been given.

A Symposium on the Magellanic Clouds was sponsored by E.S.O. in Santiago, Chile, in 1969 March in connection with the dedication of the E.S.O. Observatory at La Silla. Review papers on the Clouds have been written by Westerlund and Bok (J. R. astr. Soc. Can., 63, 105).

Gascoigne (Mon. Not. R. astr. Soc., 146, 1) has prosecuted further his P-L work on Cloud cepheids and finds V = 17·58 — 3·072 logP (7 LMC cepheids), V = 17·73 — 2·879 logP (13 SMC cepheids) while 25 SMC, 18 LMC and 13 galactic cepheids obey the Sandage relation Mv = a – 3·609 logP + 2·52(<B> – <V>).

This implies moduli of 18·73 (LMC) and 19·36 (SMC), but Gascoigne recommends, in the light of other evidence, adoption of moduli 18·75 (LMC) and 19·2 (SMC). The colours of SMC cepheids are 0.m1 bluer than those in the Galaxy and they may be somewhat underluminous for their periods.

By October 1966, the number of supernovae in extragalactic systems which had been definitely identified was 184. During the following three years an additional 70 supernovae were discovered, bringing the total number by July 1969 to 254.

The search for and the investigation of supernovae has been engaged in by an ever increasing number of observatories and astronomers, the majority of whom strongly support the continuation of the working group on supernovae and, until further notice, of the writer as its chairman.

Circular letter No. 9 was issued to the members of the working group in June 1968, giving data on the supernovae Nos. 171 to 216. A catalogue-monograph on the supernovae found since 1885 is in preparation and should be ready for presentation at the 1970 IAU Assembly. In the meantime K. Rudnicki and A. Karpowiczowa have announced that the manuscript of their catalogue of supernovae, including all major bibliographic data has been sent to the printer.

Two new approaches to the search for supernovae are in the first stages of realization, namely (a) automated searches by Professors Hynek and Colgate, the first having already resulted in 40000 bright galaxy reductions, the second being expected to start operation in the fall of 1969 and (b) the endeavours by J. Strong and F. Zwicky to have full-size objective gratings built for large (up to 50˝ apertures) Schmidt telescopes. With gratings of this type, supernovae to about mp = 16·5 may be discovered on single plates, the spectrum being observed at discovery.

• 1. A tabular presentation has been adopted and the report does not cover work in the U.S.S.R. During the report period Circular No. 6 (Report on Prague Meeting) and a Technical Supplement, No. 6a, were distributed. Work on standard luminosity profiles for a few well-observed galaxies (NGC3115, 3379, 4486) progressed more slowly than expected, but provisional results will be available for presentation at the 1970 meeting.

• 2. On the observational side the major event to report is the publication of an Atlas de Galaxias Australes by J. L. Sěrsic; it gives photographs, descriptions and detailed surface photometry of 50 southern galaxies observed with the 150-cm reflector of Cordoba Observatory. Another important contribution is the dissertation of H. D. Abies on “Optical Study of Nearby Galaxies” of interest to radio astronomy. The isophotometric atlas of 70-80 galaxies and UBV catalogue of 500 galaxies observed by G. and A. de Vaucouleurs at Mt. Stromlo and McDonald Observatories and by H. D. Abies at Flagstaff, F. Bertola at Asiago and other collaborators is being prepared for publication in 1970-71.

On the technical side the rapid development of automatic digital methods for data recording and processing should lead to a great acceleration in the production of precise isophote maps, but most commercial instruments are still deficient with respect to mechanical precision, optical design (stray light, beam definition) and electrical stability.