Interference fringes of HD 93129A for Jan. 27, 2009 FGS observations (solid line), the best fitting binary star model (dashed line), and for comparison, the fringes of an unresolved point source (dotted line). The “X” shows the location of the composite fringe that POS mode observations tracked. Owing to the roll of HST at the time of the observations, HD 93129B has a projected separation from A along the y-axis of only 0.233′′, although its true distance is 2.73′′.