A large set of data has been obtained for the Henize sample of S stars, as detailed in the present paper. Their analysis is presented in Paper III. All these data (individual measurements and spectra, as well as the various tables contained in this paper) will be available in electronic format at the internet address http://www-astro.ulb.ac.be/.
Acknowledgements
We thank Patricia Whitelock for some of the JHK observations.
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