A 36.34-day Sub-Neptune Candidate Transiting the Nearby K Dwarf TIC 402710280

Transiting sub-Neptunes on orbits longer than a month are rare around bright stars. We report a candidate with a 36.34-day period around TIC 402710280, a $V=10.0$ K dwarf at 66.8 pc that the Transiting Exoplanet Survey Satellite (TESS) has observed only in full-frame images. Four transits from 2021 to 2026, reduced by three independent pipelines, have consistent depths (mean $2092\pm 47$ ppm) and a duration of $5.27\pm 0.05$ h. Two are 36.36 days apart. The light curves exclude every shorter period consistent with both down to one day, and the duration disfavors still shorter ones, leaving $P=36.33986\pm 0.00051$ d. The candidate’s radius is $3.75\pm 0.22R_{\oplus }$ and its equilibrium temperature $498\pm 21$ K. Difference-image centroids place the signal ${1.5}^{\mathrm{\prime }\mathrm{\prime }}\pm {1.3}^{\mathrm{\prime }\mathrm{\prime }}$ from the target, and eclipse models of the only nearby Gaia source, ${3.5}^{\mathrm{\prime }\mathrm{\prime }}$ away, fit worse than a planet by $\mathrm{\Delta }{\chi }^2\ge 142$ for every main-sequence pair. TRICERATOPS gives a false-positive probability of $0.003\pm 0.004$; without high-resolution imaging, the object remains a candidate. The transit times scatter about a linear ephemeris by 21.8 min (rms), and an independent reduction reproduces the largest departure. Four transits cannot separate timing systematics from transit-timing variations. A second signal, of $407\pm 26$ ppm at 3.78 days, may come from the target, an unresolved companion, or the background star. We predict a transit in TESS Sector 114 on 2027 February 19.