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Published online by Cambridge University Press: 25 April 2016
The present account refers mainly to research work published in the period 1973–1975. Due to the limited space available, and the rapidly increasing number of contributions in the field of Commission. 28, it has not been possible to write an all-inclusive report. As in the previous IAU volumes, the report is not intended as a repetition of the summaries given in the Astronomy and Astrophysics Abstracts. In stressing the importance of certain areas of research, the personal views of the writer(s) cannot be entirely avoided.