Rossiter–McLaughlin effect

The Rossiter - McLaughlin effect is a phenomenon by which means of spectroscopy, the presence of a binary system or extrasolar planets can be detected.

Basic idea is that when a star whose equator can be seen from Earth, Doppler effects can be detected. The side of the equator, moving toward the observer, shows a blue shift, the other a red shift. At a coverage of the star by a close companion alternately changes the intensity of the blue - and red-shifted component. This allows a calculation of the orbital period, the orbital inclination and the size of the companion.