  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.
