  Many Kepler multiplanet systems have planet pairs near low-order, mean-motion
resonances. In addition, many Kepler multiplanet systems have planets with
orbital periods less than a few days. With the exception of Kepler-42, however,
there are no examples of systems with both short orbital periods and nearby
companion planets while our statistical analysis predicts ~17 such pairs. For
orbital periods of the inner planet that are less than three days, the minimum
period ratio of adjacent planet pairs follows the rough constraint P_2/P_1 >~
2.3 (P_1/day)^(-2/3). This absence is not due to a lack of planets with short
orbital periods. We also show a statistically significant excess of small,
single candidate systems with orbital periods below 3 days over the number of
multiple candidate systems with similar periods---perhaps a small-planet
counterpart to the hot Jupiters.
