AgapeZ1: a large amplification microlensing event or an odd variable star towards the inner bulge of M31

AgapeZ1 is the brightest and the shortest duration microlensing candidate e vent found to date in the Agape experiment. It occurred only 42 " from the centre of M31 at RA = 0(h) 42(m) 41.47(s) and Dcc = 41 degrees 16'39.1 " (J 2000). Our photometry shows that the half intensity duration of the event i s 5.3 days and at maximum brightness its magnitude is R = 17.9 (M-R similar to -6) with B - R = 0.80 mag colour. A search on HST archives produced a s ingle resolved star within the projected event position error box. Its magn itude is R = 22, and its colour is compatible with that of the event at the 2 sigma level. If this event is identified with the HST star, the implied amplification is about 4 magnitudes or 40 in brightness. This would lead to an Einstein cro ssing time radius of about 60 days. AgapeZ1 could be a bulge/bulge microlen sing event involving a binary star. The photometric properties of the object are inconsistent with classical M3 1 variable stars such as Miras, classical novae, dwarf-novae, and bumpers. However, we cannot rule out the possibility that AgapeZ1 is in fact an odd variable star.