  If a transiting exoplanet has a moon, that moon could be detected directly
from the transit it produces itself, or indirectly via the transit timing
variations it produces in its parent planet. There is a range of parameter
space where the Kepler Space Telescope is sensitive to the TTVs exomoons might
produce, though the moons themselves would be too small to detect
photometrically via their own transits. The Earth's Moon, for example, produces
TTVs of 2.6 minutes amplitude by causing our planet to move around their mutual
centre of mass. This is more than Kepler's short-cadence interval of 1 minute
and so nominally detectable (if transit timings can be measured with comparable
accuracy), even though the Moon's transit signature is only 7% that of Earth's,
well below Kepler's nominal photometric threshold. Here we examine several
Kepler systems, exploring the hypothesis that an exomoon could be detected
solely from the TTVs it induces on its host planet. We compare this with the
alternate hypothesis that the TTVs are caused by an non-transiting planet in
the system. We examine 13 Kepler systems and find 8 where both hypotheses
explain the observed TTVs equally well. Though no definitive exomoon detection
can be claimed on this basis, the observations are nevertheless completely
consistent with a dynamically stable moon small enough to fall below Kepler's
photometric threshold for transit detection, and these systems warrant further
observation and analysis.
