Binary stars of the ζ Aurigae type (eclipsing systems containing a cool giant plus a hot main-sequence star) offer a unique and highly effective method of probing a stellar chromosphere. Close to occultation the main-sequence star acts as a light probe behind the giant's chromosphere, enabling an observer to detect changing conditions in that chromosphere along the line of sight. The technique is powerful, the effects dramatic. However, presently known eclipsing systems number only about 10, and a much greater sample is required for meaningful statistics of the properties of stellar chromospheres. New surveys of fainter binaries should be investigated for eclipses in order to gain more information on chromospheres in general. Such information is vital for modelling stellar photospheres, from which abundances are derived. This paper describes the very different behaviour of chromospheric material in three 3rd-magnitude giants.
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