A Candidate Low-mass Disk-eclipsing Binary in the 316 Myr Open Cluster UPK 13

UPK 13-c2 is a candidate member of the about 316 Myr open cluster UPK 13 and was previously classified as a white dwarf + main-sequence (WD + MS) binary with 99.44% confidence. We present multi-band photometric evidence that it is instead more plausibly a late-K/early-M binary with a misaligned circumbinary disk. The photometry reveals a flat-bottomed eclipse at P=36.71 days with an approximately achromatic about 40% flux decrement from the optical through W1, a reduced W2 depth, and a prominent mid-infrared excess at W3/W4. Two independent diagnostics strongly disfavor the WD + MS interpretation. First, the flat eclipse floor and 2.5-day ingress require complete occultation of an extended stellar component; a white dwarf would cross the disk edge in 2 hr and cannot naturally reproduce the observed multi-day trapezoid. Second, the difference spectrum is well fit by a single about 4000 K thermal spectral energy distribution, favoring a late-K/early-M dwarf over a white dwarf as the occulted source. A decoupled SED decomposition yields a template-based late-K/early-M binary estimate of M1V+K9V with M tot about1.4,M and an overall systematic uncertainty of about 20%. Under a sharp-edge eclipse model, forward modeling of the light curve favors an eccentric, spatially localized occulting structure. The light-curve morphology places UPK 13-c2 in the same geometric class as KH 15D and Bernhard-2. If cluster membership is confirmed, UPK 13-c2 may be the oldest known main-sequence disk-eclipsing binary.