Where are the binary source galactic microlensing events?

Among the galactic microlensing events detected by the 4 observing groups EROS (Aubourg et al. 1993), MACHO (Alcock et al. 1993, 1997a,b), OGLE (Udalski et al. 1994a-e) , and DUO (Alard et al. 1995a,b), some events show the characteristics of a binary lens, namely MACHO LMC#1 (Dominik & Hirshfeld 1994, 1996), OGLE#7 (Udalski et al. 1994d), DUO#2 (Alard et al. 1995b), MACHO LMC#9 (Bennett et al. 1996), MACHO Bulge 95-12 (Pratt et al. 1995), and MACHO Bulge 96-3 (Stubbs et al. 1997). In contrast, no event has been claimed to involve a binary source, though Griest & Hu (1992) have predicted that around 10% of the events should involve features of a binary source. Given this situation, one may clearly pose the question where the binary source microlensing events are. Here I argue that the lack of claimed binary source microlensing events may be due to the fact that most of the light curves for such events can successfully be explained with the simpler model of a blended single source. In fact, such a model is successful for the event OGLE#5 (Dominik 1996; Alard 1997) and most of the DUO events also involve blending (Alard 1997). In this paper, I discuss also a model with a binary source for OGLE#5 and compare it with the model involving a blended single source.