Distinguishing between Planetary and Binary Interpretations of   Microlensing Central Perturbations under Severe Finite-Source Effect
  In the current strategy of microlensing planet searches focusing on high-magnification events, wide and close binaries pose important sources of contamination that imitates planetary signals. For the purpose of finding systematic differences, we compare the patterns of central perturbations induced by a planet and a binary companion under severe finite-source effect. We find that the most prominent difference shows up in the morphology of the edge features with negative excess that appear at the edge of the circle with its center located at the caustic center and a radius equivalent to the source radius. It is found that the feature induced a binary companion forms a complete annulus, while the feature induced by a planet appears as several arc segments. This difference provides a useful diagnostic for immediate iscrimination of a planet-induced perturbation from that induced by a binary companion, where the absence of a well-developed dip in the residual from the single-lensing light curve at both or either of the moments of the caustic center's entrance into and exit from the source star surface indicates that the perturbation is produced by a planetary companion. We find that that this difference is basically caused by the difference between the shapes of the central caustics induced by the two different types of companions. 
Cheongho Han