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The Rossiter-McLaughlin effect (RM effect, or just RM) is a spectroscopic effect on stars during transits, that reveals whether the transiting object (extra-solar planet or binary companion) has a retrograde orbit (i.e., opposite to the star's own rotation). The effect was first detected in 1924 for binary stars.
A bit of a rotating star's light is redshifted and a bit is blueshifted, since some is coming from surface rotating toward us and some from the other side, rotating away from us. A transit blocks part of the starlight, the relative amounts of shifted light in our view depending upon which portion of the transit is in progress. The order of which shift dominates earlier during the transit reveals whether the orbit is prograde or retrograde, "blueshift earlier" indicating a retrograde orbit.