1999ApJ...527..167Z

We propose an observational test that can break the indeterminacy of two main classes of microlensing models of the Magellanic Clouds: (a) the lenses are located in the Galactic halo, and (b) the lenses are located in the LMC disk. The source stars in the latter (self-lensing) models tend to be at the far side or behind the LMC disk, thus experiencing more reddening and extinction by dust in the LMC disk than ordinary stars in a nearby line of sight. Clearly, such bias would not occur in the MACHO halo lensing models. We show that this reddening effect is at a level readily observable for the present 30 or so microlensing alerts fields, either with multiband photometry from a good seeing site, or more definitively with ultraviolet (UV) spectroscopy with Space Telescope Imaging Spectrograph. Stars behind the LMC dust layer should stand out as UV-faint objects (by more than 1 mag than average stars in the LMC). HST can also resolve numerous faint neighboring stars within a few arcseconds of a lensed source, hence removing blending in these crowded regions and building a reddening map to control the patchiness of dust.