This volume contains the papers and discussions at IAU Colloquium No. 21 on Variable Stars in Globular Clusters and in Related Systems held in Toronto on the 29th, 30th and 31st August 1972. It was the intention of the organizers that this meeting should honour the life long work in this field of Professor Helen Sawyer Hogg. She has been continuously active in observational research on variables in globular clusters for 46 years and her catalogues and bibliographies as well as her research papers, review articles and IA U reports as chairman of the committee on variable stars in clusters are of fundamental importance to all workers in this field. The scope of the colloquium covered both observational and theoretical aspects of the problem, including the relationship of variables to non-variable cluster members, the position of the variables in the HR diagram and their importance for problems of stellar evolution, empirical data on the variables, periods and period changes, and the relevant parts of pulsation theory. The meeting was particularly successful in bringing together observers and theorists. It will have achieved its object if it has shown both observers and theorists which are the problems most suitable for attack at the present time. The meeting clearly demonstrated the great importance of research on variables in globular clusters and related systems for our understanding both of stellar evolution and stellar pulsation.

Inhalt
I/General Problems of Variables in Population II Systems.- Variables in Globular Clusters, the Common and the Rare (Review Paper).- Some Problems in the Study of Variable Stars in Globular Clusters.- Observations of Faint Globular Clusters.- Planetary Nebulae, Globular Clusters and the Galactic Halo.- An Abundance Analysis of Fehrenbach's Star (HD 116745) in Omega Centauri.- On the Nature of the 41-Day Cycle of RR Lyrae.- Variable Stars in Dwarf Spheroidal Galaxies (Review Paper).- II/RR Lyrae Variables in Population II Systems.- Observational Aspects of RR Lyrae Variables in Globular Clusters (Review Paper).- New Variable Stars in the Globular Cluster NGC 6401.- Variable Stars in NGC 4590.- Two-Colour Photometry of RR Lyrae Variables in NGC 6981.- Intermediate Band Photometry of RR Lyrae Variables in ? Cen and 47 Tuc.- UBV Magnitudes and Colours of 62 RR Lyrae Stars in ? Centauri (NGC 5139).- Two-Color Observations of RR Lyrae Stars in M14.- On the Changes of Periods of RR Lyrae Variables in the Globular Clusters M5, M53 and NGC 5053.- Period and Period Changes of RR Lyrae Variables in M15.- Period Changes as Evidence for Evolution in ? Cen.- Short Period Variables in the Magellanic Cloud Cluster NGC 1466.- The RR Lyrae Stars in the Magellanic Clouds.- III/Slow Variables in Population II Systems.- Observational Aspects of Slow Variables in Globular Clusters (Review Paper).- A Preliminary Investigation of Period Changes for W Virginis Stars in Globular Clusters.- Mean Magnitudes and Colours of Six Cepheids in Three Red Globular Clusters of the Large Magellanic Cloud.- Red Variable Stars in Metal Rich Globular Clusters.- Narrow-Band Photometry of Red Variables in Globular Clusters.- The Variable Stars in NGC 6723.- The Period-Luminosity Relation for Cepheids inGlobular Clusters.- IV/Theoretical Considerations of Population II Variables.- Variable Stars and Evolution in Globular Clusters (Review Paper).- On the Two Oosterhoff Groups of Globular Clusters.- On the Interpretation of RR Lyrae Properties in Globular Clusters and in Other Population II Systems.- Possible Evolutionary Interpretation of the Dependences on the Diagrams for RR Lyrae Variables.- The Evolutionary Status of Population II Cepheids.- Semiconvection and the RR Lyrae Variables.- The Helium Abundance in the Envelopes of the Bluest RR Lyrae Stars in Globular Clusters and Dependence of Globular Cluster Variable Star Properties on Chemical Composition.