Xallarap is a variation in a gravitational lensing observation caused by the orbital motion of the source.
A more traditional and similar effect, parallax, is the variation caused by motion of the Earth around the Sun.
[3] However, modern surveys such as the Optical Gravitational Lensing Experiment (OGLE) observe millions of stars each night, and see microlensing many times each year.
Traditionally in astronomy, a change in view caused by the Earth's motion is called parallax, and this is the term used by researchers for this effect.
However, if the source star is part of a binary system, then it too has orbital motion, and this can modify the alignment just as the Earth's movement can.