A Single-transit Planet Candidate of about 3 Earth Radii around the G dwarf TIC 405408675

I report one transit-like event in TESS photometry of the G dwarf TIC 405408675 (HIP 108342; T = 8.21, 69 pc) in Sector 82: depth 749 ppm, duration 7.3 h. No second transit is seen in Sector 55. The dip is present in the raw pixels and survives an independent sky subtraction, and a difference image favours the target over its catalogue neighbours. If it orbits the target, the transiting object has a radius of 3.2 Earth radii; a central transit on a circular orbit would need a period of at least 45 days. At the orbital periods the TESS data still allow, sampled from 15 to 1001 days, TRICERATOPS gives median false-positive probabilities of 0.008 to 0.11 where the duration also fits a circular orbit, with single runs up to 0.59; this is not a statistical validation. I publish it so that observers can look for a second transit. Preprint of a Research Note of the AAS, with the data behind it: the light curve around the event, the transit fit, the period scan and the pixel tests.