The PLANET collaboration has detected an anomaly in the on-going microlensing event OGLE 2000-BUL-28. This event was announced by OGLE as a "Very short duration event, first caustic of binary lens or variable star". Recently reduced PLANET data from two observing sites appear to suggest that this event is due to multiple lens microlensing. Two caustic crossings may have occured at JD-2450000= 1677 and JD-2450000= 1682. An electronic alert was issued by PLANET. (The original alert contained contained a typographical error that placed the second possible crossing incorrectly at JD-2450000= 1680.)
From JD-2450000= 1682 through JD-2450000= 1688, the light curve is declining as might be expected after the exit of a caustic region. The event may be blended, though a complete analysis of the full data set will be required to substantiate this claim. PLANET continues to observe this event.
RA (J2000): 17:58:13.12
Dec (J2000): -29:09:14.3
The baseline I magnitude reported by the OGLE team is 16.98.
A postscript finding chart can be downloaded here.
Caustic crossing events can be used to measure source resolution effects, which can allow the determination of the relative proper motion of the lens and, in special circumstances, the limb-darkening of the source star. High magnification events, are especially likely to betray the presence of possible planets orbiting the lens.
Finding charts and future updated information can be found on the PLANET web site at http://planet.iap.fr.
OGLE information on this event can be found at the OGLE alert web page at http://www.astrouw.edu.pl/~ftp/ogle/ogle2/ews/2000/bul-28