Note to users. If you're seeing this message, it means that your browser cannot find this page's style/presentation instructions -- or possibly that you are using a browser that does not support current Web standards. Find out more about why this message is appearing, and what you can do to make your experience of our site the best it can be.
Rapidly rotating stars are flattened spheres with poles that are hotter than the equator.
The author is at the Landessternwarte Königstuhl, University of Heidelberg, D-69117 Heidelberg, Germany. E-mail: a.quirrenbach{at}lsw.uni-heidelberg.de