What does a spectroscopic binary show periodic variations in?

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A spectroscopic binary shows periodic variations in its radial velocity, as the two stars orbit around a common center of mass. This can be detected through observations of the star's spectral lines, which will shift back and forth as the stars move towards and away from us. Proper motion refers to the apparent motion of a star across the sky, and is not directly related to a binary companion. Brightness and spectral type may also vary in a binary system, but they do not show the same periodicity as radial velocity.

What Does a Spectroscopic Binary Show Periodic Variations In