  Torques from a mutually inclined perturber can change a transiting planet's
impact parameter, resulting in variations in the transit shape and duration.
Detection of and upper limits on changes in impact parameter yield valuable
constraints on a planetary system's three dimensional architecture. Constraints
for warm Jupiters are particularly interesting because they allow us to test
origins theories that invoke a mutually inclined perturber. Because of warm
Jupiters' high signal-to-noise transits, changes in impact parameter are
feasible to detect. However, here we show that allowing the impact parameter to
vary uniformly and independently from transit to transit leads to incorrect
inferences about the change, propagating to incorrect inferences about the
perturber. We demonstrate that an appropriate prior on the change in impact
parameter mitigates this problem. We apply our approach to eight systems from
the literature and find evidence for changes in impact parameter for warm
Jupiter Kepler-46b. We conclude with our recommendations for light curve
fitting, including when to fit impact parameters vs. transit durations.
