Rossiter–McLaughlin effect

When the secondary star or planet transits the primary, it blocks part of the latter's disc, preventing some of the shifted light from reaching the observer.

As the transiting object moves across to the other side of the star's disc, the redshift anomaly will switch from being positive to being negative, or vice versa.

The amplitude of the redshift anomaly depends on the mutual alignment between the primary star's equator and the secondary's path of transit.

For determination of the three-dimensional true obliquity (typically indicted with ψ), the projected obliquity needs to be combined with the values of inclination of the primary's equator obtained by comparing the projected rotational velocity with the true rotation period, and of the secondary's orbit obtained from the impact parameter of transit and/or doppler spectroscopy, which instead measure the inclination angle relative to the line of sight.

This effect has been used to show that as many as 25% of hot Jupiters are orbiting in a retrograde direction with respect to their parent stars, strongly suggesting that dynamical interactions rather than planetary migration produce these objects if no additional processes are involved.

Animation of the Rossiter-Mclaughlin (RM) effect
The viewer is situated at the bottom. Light from the anticlockwise-rotating star is blue-shifted on the approaching side, and red-shifted on the receding side. As the planet passes in front of the star it sequentially blocks blue- and red-shifted light, causing the star's apparent radial velocity to change, but it does not in fact change.