  Transit search programs such as CoRoT and Kepler now have the capability of
detecting planets as small as the Earth. The detection of these planets however
requires the removal of all false positives. Although many false positives can
be identified by a detailed analysis of the LCs, the detections of others
require additional observations. An important source of false positives are
faint eclipsing binaries within the PSF of the target stars. We develop a new
method that allows us to detect faint eclipsing binaries with a separation
smaller than one arcsec from target stars. We thereby focus on binaries that
mimic the transits of terrestrial planets. These binaries can be either at the
same distance as the target star (triple stars), or at either larger, or
smaller distances. A close inspection of the problem indicates that in all
relevant cases the binaries are brighter in the infrared than in the optical
regime. We show how high resolution IR spectroscopy can be used to remove these
false positives. For the triple star case, we find that the brightness
difference between a primary and an eclipsing secondary is about 9-10 mag in
the visual but only about 4.5-5.9 magnitudes in the K-band. We demonstrate how
the triple star hypothesis can be excluded by taking a high-resolution IR
spectrum. Simulations of these systems show that the companions can be detected
with a false-alarm probability of 2%, if the spectrum has a S/N-ratio > 100. We
show that high-resolution IR spectra also allows to detect most of the false
positives caused by foreground or background binaries. If high resolution IR
spectroscopy is combined with photometric methods, virtually all false
positives can be detected without RV measurements. It is thus possible to
confirm transiting terrestrial planets with a modest investment of observing
time.
