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Published online by Cambridge University Press: 25 April 2016
This Comm. 20 triennial report is prepared as a short summary of the scientific progress in the fields of interest, where, by necessity, only a few highlights are to be mentioned. On the Comm. 20 webpage, the interested reader may find a more complete list of recent references to the relevant literature. Its URL at the time of writing (October 1999) is www.astro.uu.se/IAU/c20/, but mirror sites are expected to have been set up at the time of appearance of these Transactions.