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                        PLANET ANOMALY ALERT
		               2004 #3
                      (issued 2-Jun, 13:40 UT) 
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               OGLE 2004-BLG-273 undergoing caustic exit 



Dear Colleagues,

while OGLE data on the event OGLE 2004-BLG-273 collected until HJD-2450000 
= 3154.65 looked consistent with microlensing of a point source by a point-
like lens, where the last point reached a magnification of ~ 1.7 mag above
baseline, a nearly constant lightcurve has been observed between
HJD-2450000 = 3156.5 and HJD-2450000 = 3158.0 at about 3 mag above
baseline.

PLANET observations at the Danish 1.54m telescope at ESO LaSilla (Chile),
which have been carried out by C. Coutures and C. Vinter, show a rise by
0.6 mag over 1.1 days, indicating an approach to a fold-caustic exit and
therefore revealing the binary nature of the lens. 

The center of the source is expected to cross the caustic at HJD-2450000 =
3159.28 - 0.09 + 0.14, i.e. 2-Jun, 18.43 UT -2.2 h +3.4h. This estimate is
based on the assumption of a point source. Current data does not allow to
determine the source size. Please be aware of the fact that the source 
begins to exit the caustic before the estimated crossing time of its center.
The caustic exit may therefore already be in progress.
    
Frequent photometric measurements during the course of the caustic exit can 
be used to obtain a measurement of the proper motion between lens and source 
and allow to measure the brightness profile of the source. A good coverage 
will reduce uncertainties in the properties of the binary lens system
and in particular allow a lens mass measurement if parallax effects can
be assessed properly. 

Event:            OGLE 2004-BLG-273
RA (J2000):       17:47:14.67  
Dec (J2000):      -35:43:57.3 

The baseline I magnitude reported by the OGLE team is 19.683.

More information is available from PLANET at 'http://planet.iap.fr' or from
OGLE at 'http://www.astrouw.edu.pl/~ftp/ogle/ogle3/ews/2004/blg-273.html'. 

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On behalf of the PLANET collaboration,

Martin Dominik

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