On event OGLE 2005-BLG-331, the OGLE team reported a sudden strong rise in magnification exceeding 1 mag/hr and considered the possibility that they have caught a caustic entry in progress. By issuing a secondary alert, they enabled us to obtain data on this event during the same night with the Danish 1.54m at ESO LaSilla. As reported in PLANET anomaly alert 2005 #6 (issued 2-Jul, 11:55 UT), these data confirm the rise and together with more OGLE data support the interpretation of a fold-caustic entry. However, our data can only serve as an indication since we were not able to obtain a good reference frame yet and the weather conditions were far from optimal.
A fold-caustic entry model would prefer a caustic passage time of (6 +/- 2) hrs, with the leading limb entering at HJD=2453553.712 +/- 0.003 (2-Jul, 5:05 UT). This would mean that the caustic passage is about to finish as the PLANET alert was circulated, with the trailing limb entering. In addition to the entry of the leading limb, this is a second phase of strong value for the measurement of limb darkening.
After further OGLE data started to show a separate rise after the suspected fold-caustic entry, which was initially expected to be related to the corresponding exit, subsequent observations by PLANET with the Danish 1.54m at ESO LaSilla as well as by OGLE reveal another fold-caustic entry happening before a caustic exit related to the first entry was observed. This was reported in PLANET anomaly update 2005 #6-1 (issued 4-Jul, 4:45 UT).
Possible interpretations for the observed data could be a source binary passing behind a binary lens or a triple lens system.
In PLANET anomaly update 2005 #6-2 (issued 18-Jul, 0:05 UT), we reported that after an initial rise of about 0.3 mag between HJD=2453562.5 and HJD=2453565.5 (11-Jul until 14-Jul), PLANET data from the Canopus 1.0m near Hobart (Tasmania) and the Danish 1.54m at ESO LaSilla (Chile), in agreement with OGLE data, show the target to be approximately 1.2 mag fainter at the time of the alert. These data leave some room for one or two caustic exits between HJD=2453565.7 (14-Jul, 5:00 UT) and HJD=2453566.9 (15-Jul, 9:30 UT), when observations at several of our sites were prevented by bad weather.
A light curve with the latest data is available.