The Optical Gravitational Lensing Experiment. OGLE #7: Binary Microlens or a New Unusual Variable?

We present the light curve of an unusual variable object, OGLE #7, detected during the OGLE search for microlensing events. After one season of being in a low, normal state, the star brightened by more than 2~mag with a characteristic double-maximum shape, and returned to normal brightness after 60 days. We consider possible explanations of the photometric behavior of OGLE #7. The binary microlens model seems to be the most likely explanation -- it reproduces well the observed light curve and explains the observed colors of OGLE #7. The characteristic time scale of the OGLE #7 event, $t_E$, is equal to 80 days, the longest observed to date. The binary microlens model predicts that the spectrum of the star should be composite, with roughly 50% of its light in the $I$-band coming from a non-lensed source.