Extinction bias of microlensed stars towards the LMC and the fraction of machos in the halo
We study the effect of reddening on microlensed stars towards 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 neighbouring lines of sight. We simulate this effect in a variety of models for the LMC stars and dust. We stress that the optical depth is not one number, but 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 controled 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 ultra-violet. The non-detection of the reddening bias would be strongly in favor of machos in the Galactic halo.