  In this paper, we explore the astrophysical implications of near-field
microlensing and its effects on stellar transit observations, with a special
emphasis on the Kepler mission. Kepler is a NASA-approved mission whose goal is
to detect a large number of extrasolar, earth-like planets by obtaining
near-continuous photometry of > 100,000 F, G, and K dwarfs for four years. The
expected photometric precision of Kepler is 90 micromag (achieved in 15 minute
samples), at which the effect of microlensing by a transiting companion can be
significant. For example, for a solar-type primary transited by a white-dwarf
secondary, the maximum depth of the transit is 0.01%, which is almost entirely
compensated by the microlensing amplification when the white dwarf is at ~0.05
AU. The combined effect of microlensing and transit increases to a net
amplification of 150 micromag at an orbital separation of 0.1 AU, and 2.4
millimag at an orbital separation of 1 AU. Thus, the effect of microlensing can
be used to break the degeneracy between a planetary-mass object for which the
microlensing effect is negligible, and a more massive object of the same size.
For brown dwarfs at orbital separations of a few AU, the effect of microlensing
is several percent of the transit depth, and hence the microlensing effect must
be taken into account in deriving the physical parameters of the brown dwarf.
The microlensing signal caused by a neutron star or a black hole in a binary
can be several millimag, far exceeding the transit depth, and potentially
detectable even from ground-based observations. Kepler will be sensitive to
white dwarfs, neutron stars, and black holes in binaries through their
microlensing signatures. These observations can be used to derive the frequency
of such compact objects in binaries, and to determine their masses.
