Extinction bias of microlensed stars toward the large Magellanic Cloud and the fraction of MACHOs in the halo

We study the effect of reddening on microlensed stars toward the LMC. If lenses are in the LMC disk, then the source stars should be at the far side or behind the LMC disk. Thus, they should experience more reddening and extinction by dust in the LMC disk than typical stars in the immediate neighboring lines of sight. We simulate this effect in a variety of models for the LMC stars and dust. We stress that the microlensing optical depth is a function of the reddening of the survey stars. We discuss how these effects could be used to constrain the fraction of MACHOs in the dark halo. The effect of patchiness of dust can be controlled by working with faint stars in the smallest patches of the sky around the microlensed stars. This can be done most effectively with the Hubble Space Telescope in the ultraviolet. The nondetection of the reddening bias would be strongly in favor of MACHOs in the Galactic halo.