A Candidate 1.15 Earth-radius Planet on a 4.21-day Orbit around TOI-6000 from Archival TESS Photometry

I report TIC 259233660.02, a candidate second transiting planet around the M dwarf TOI-6000, host of the validated ultra-short-period planet TOI-6000 b. A box least-squares search of TESS full-frame-image light curves found the signal, and the 2-minute photometry of 33 sectors, with TOI-6000 b masked, recovers a 781±88 ppm, 1.5 hr transit every 4.213442 days (signal-to-noise ratio 8.8; 6.8 with a robust estimator). The implied radius is 1.15±0.09 R⊕. The signal lies below the SPOC detection threshold. A point-spread-function fit to difference images from 39 sectors places the source 2.0±2.0″ from the target and disfavors every Gaia star within 63″. TRICERATOPS gives a false-positive probability of 0.089, or 0.049 when the pixel-excluded neighbors are removed. The depth declines with time at 1.8–2.7σ. I provide an ephemeris for ground-based confirmation.