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ABSTRACT Transit timing variations (TTVs) can be induced by a range of physical phenomena, including planet–planet interactions, planet–moon interactions, and stellar activity. Recent work has shown that roughly half of moons would induce fast TTVs with a short period in the range of 2–4 orbits of its host planet around the star. An investigation of the Kepler TTV data in this period range identified one primary target of interest, Kepler-1513 b. Kepler-1513 b is a $$8.05^{+0.58}_{-0.40}$$ R⊕ planet orbiting a late G-type dwarf at $$0.53^{+0.04}_{-0.03}$$ au. Using Kepler photometry, this initial analysis showed that Kepler-1513 b’s TTVs were consistent with a moon. Here, we report photometric observations of two additional transits nearly a decade after the last Kepler transit using both ground-based observations and space-based photometry with TESS. These new transit observations introduce a previously undetected long period TTV, in addition to the original short period TTV signal. Using the complete transit data set, we investigate whether a non-transiting planet, a moon, or stellar activity could induce the observed TTVs. We find that only a non-transiting perturbing planet can reproduce the observed TTVs. We additionally perform transit origami on the Kepler photometry, which independently applies pressure against a moon hypothesis. Specifically, we find that Kepler-1513 b’s TTVs are consistent with an exterior non-transiting ∼Saturn mass planet, Kepler-1513 c, on a wide orbit, $$\sim 5~{{\ \rm per \, cent}}$$ outside a 5:1 period ratio with Kepler-1513 b. This example introduces a previously unidentified cause for planetary interlopers in the exomoon corridor, namely an insufficient baseline of observations. 

ABSTRACT White dwarfs are one of the few types of stellar object for which we have yet to confirm the existence of companion planets. Recent evidence for metal contaminated atmospheres, circumstellar debris discs, and transiting planetary debris all indicates that planets may be likely. However, white dwarf transit surveys are challenging due to the intrinsic faintness of such objects, the short time-scale of the transits, and the low transit probabilities due to their compact radii. The Large Synoptic Survey Telescope (LSST) offers a remedy to these problems as a deep, half-sky survey with fast exposures encompassing approximately 10 million white dwarfs with r < 24.5 apparent magnitude (mr). We simulate LSST photometric observations of 3.5 million white dwarfs over a 10 yr period and calculate the detectability of companion planets with P < 10 d via transits. We find typical detection rates in the range of 5 × 10−6 to 4 × 10−4 for Ceres-sized bodies to Earth-sized worlds, yielding ∼50–$4000$ detections for a 100 per cent occurrence rate of each. For terrestrial planets in the continuously habitable zone, we find detection rates of ∼10−3 indicating that LSST would reveal hundreds of such worlds for occurrence rates in the range of 1–10 per cent.