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Published online by Cambridge University Press: 04 August 2017
A considerable amount of observational information has been gathered on the so-called Hubble-Sandage Variables or S Doradus variable stars, largely due to the efforts of the Heidelberg group (see Wolf, this volume for a review). It is believed that the star P Cygni also belongs to this category. This group of objects is now being referred to as Luminous Blue Variables because they are massive (>10 M⊙), evolved (spectral type B or later) objects showing variability of more than one kind, and evidence of nuclear processed material at the surface.