  We attempt to determine whether the MACHO microlensing source stars are drawn
from the average population of the LMC or from a population behind the LMC by
examining the HST color-magnitude diagram (CMD) of microlensing source stars.
We present WFPC2 HST photometry of eight MACHO microlensing source stars and
the surrounding fields in the LMC. The microlensing source stars are identified
by deriving accurate centroids in the ground-based MACHO images using
difference image analysis (DIA) and then transforming the DIA coordinates to
the HST frame. We consider in detail a model for the background population of
source stars based on that presented by Zhao, Graff & Guhathakurta. In this
model, the source stars have an additional reddening <E(B-V)> = 0.13 mag and a
slightly larger distance modulus <Delta mu> ~ 0.3 mag than the average LMC
population. We also investigate a series of source star models, varying the
relative fraction of source stars drawn from the average and background
populations and the displacement of the background population from the LMC. Due
to the small number of analyzed events the distribution of probabilities of
different models is rather flat. A shallow maximum occurs at a fraction s_LMC ~
0.8 of the source stars in the LMC. This is consistent with the interpretation
that a significant fraction of observed microlensing events are due to lenses
in the Milky Way halo, but does not definitively exclude other models.
