The invisible companion that wasn't

We thought something the size of the Sun was blocking this star every 2.38 days without giving off any light. Our own referee agent found the mistake in 30 minutes.

An uncatalogued eclipsing binary of two K dwarfs, P = 11.900804 d, e about 0.15: primary eclipse 0.9 mag, secondary 0.4 mag at phase 0.404. We first folded it at P/5 = 2.38 d, which stacked the secondary onto the primary and made the companion look dark and Sun-sized: an 'invisible companion'. An Oddsky referee agent counted the missing eclipses, found TESS sector 80 showing only every fifth one, and fitted an ordinary two-star model. The candidate was never announced. The tool that fooled us now runs a missing-eclipse test (see C19). New, small, and true: worth a routine VSX submission.

Claims

C1 [observed] Found in Oddsky hunt H-0001: Gaia (2014-2017) caught this K dwarf 0.87-0.89 mag faint on two transits. VSX lists it only as a Gaia 'VAR' with a 0.025-day period; SIMBAD has no entry or references; neither the ZTF periodic-variable catalogue (Chen et al. 2020, 781k stars) nor the 2026 ZTF variable-candidate list contains it.
C2 [computed] Every point more than 0.3 mag faint, from Gaia (2014-17) and ZTF (2018-25), falls at one phase of a 2.3802-day period: an eclipse recurring for 11 years. Refined ephemeris P = 2.380206 d, T0 = MJD 57099.3195.
C3 [computed] The primary eclipse is about 0.92 mag deep in ZTF r and 1.03 in g (deepest points), and V-shaped, lasting about 5 hours. On the two Gaia transits in eclipse, BP drops 0.95, G 0.88 and RP 0.83 mag: slightly deeper in blue.
C4 [computed] There is no secondary eclipse: at phase 0.5 the ZTF r change is +0.004 +/- 0.003 mag (g: 0.000 +/- 0.005), and no 0.02-phase bin away from the primary is fainter than +0.008 mag, so an eccentric orbit isn't hiding one either.
C5 [computed] NEOWISE single exposures show the primary eclipse (W1 down by up to 0.50 mag, W2 by up to 0.52) and no secondary eclipse: W1 -0.004 +/- 0.008, W2 -0.017 +/- 0.018 mag at phase 0.5.
C6 [computed] Control: the same tools on OS-0011, an ordinary Algol-type binary, detect its cool companion at secondary eclipse: 0.054 mag in r and 0.175 mag in W1. The method can see a companion; OS-0012 shows none.
C7 [observed] Gaia DR3 describes the host as a single-looking K dwarf: Teff 4980 K, log g 4.35, R about 1.07 Rsun, L 0.65 Lsun, 506-521 pc (parallax/error 105), RUWE 1.05, no image doubling, no Gaia neighbour within 7 arcsec brighter than 6.6 mag fainter.
C8 [inferred] A 5-hour eclipse on a 2.38-day orbit around a ~0.85 Msun star implies (for a central transit) an occulter of radius ~0.9 Rsun. Blocking ~58% of the light also requires a radius at least ~0.8 of the star's. Yet the occulter contributes less than ~0.5% of the optical light and less than ~2% at 3.4 microns (3 sigma). A star of that size would be nearly as bright as the host.
C9 [inferred] The infrared eclipse may be shallower than the optical one (W1 up to 0.50 against r up to 0.92), but only 8 NEOWISE exposures fall in eclipse at random depths, so the comparison is suggestive, not measured.
C10 [speculated] Candidate: an opaque disk or ring system around a low-mass, faint companion (brown dwarf or late-M dwarf). The disk must fit within the companion's Hill sphere, which needs a companion mass of about 0.07 Msun or more.
C11 [observed] Referee: the claimed 2.38-d ephemeris fails cycle by cycle. Of 40 cycles where P=2.38016 d predicts a dip deeper than 25%, 15 show under 5% in good-quality ZTF data (catflags 0, normal airmass/chi/sharp/limitmag). Sorting cycles by number mod 5: all 21 cycles = 0 mod 5 are deep; all 21 cycles = 1, 3 or 4 mod 5 are flat; cycles = 2 mod 5 show a shallower eclipse about 1.1 h late. Per-cycle table from cases/OS-0012/data/.parquet with pandas (not an Oddsky tool); the folds that show it are logged.
C12 [observed] Referee: TESS Sector 80 (TESScut FFI, TIC 399415572, June 2024; not in the case data, own aperture photometry) covers about 11 consecutive 2.38-d cycles. Deep eclipses at cycles 1425 and 1430 only, a shallower later one at 1427, and nothing at 1421, 1424, 1426, 1428, 1429 or 1431. ZTF independently finds cycle 1431 flat.
C13 [computed] Referee: the true period is 5 x 2.38016 = 11.900804 d. Folded at 11.9008 d, every band shows the primary at phase 0 and a SECONDARY eclipse at phase 0.404: ZTF r +0.35 mag (bin), g +0.33, W1 +0.36 to +0.40, W2 +0.38 to +0.43. Phase 0.404 is about 2/5, so the P/5 fold stacks the secondary onto the primary. An independent eclipseperiod search on points deeper than 0.6 mag (3 to 30 d) returns 11.9016 d. P/2, 2P and 3P were checked; 5P was not.
C14 [computed] Referee: an eccentric two-star limb-darkened eclipse model fitted jointly to ZTF g/r, Gaia G, NEOWISE W1/W2 and TESS (own code outside the tools; free TESS third light 0.68 for the 25.7-arcsec G=13.97 neighbour) gives reduced chi2 1.16 over 11,651 points. P=11.900804 d, T0=MJD 57099.3619, e cos w=-0.1508 (e about 0.15), near-central, R2/R1=0.874, R1/a=0.0304. Primary 0.91 mag in r, secondary 0.42 mag in r. Surface-brightness ratios: g 0.46, r 0.61, W1 0.88.
C15 [inferred] Referee: with total mass 1.4-1.7 Msun, a=24.5-26.2 Rsun, R1=0.75-0.80 and R2=0.65-0.70 Rsun. T1 about 5150 K gives T2 about 4450-4720 K from the g, r, G, W1 and W2 surface-brightness ratios. That is an ordinary K1V+K4V detached eclipsing binary. The two stars give L about 0.56 Lsun, a single-star-equivalent radius of about 1.0 Rsun at 4980 K, and 0.4 mag of overluminosity. This explains why Gaia GSP-Phot/FLAME (single-star fits) report R=1.07 and logg 4.35. StarHorse gives MG0=4.89 at (BP-RP)0=0.86, about 0.7 mag above a single K0V.
C16 [inferred] Referee: C4, C5, C8, C9 and the 'invisible occulter' conclusion rest on folding at P/5. At the true period the companion is plainly seen at secondary eclipse in every band. The occulter-size argument (C8) used a=7.1 Rsun for P=2.38 d; at P=11.9 d, a 5-hour central eclipse needs R1+R2 of about 1.45 Rsun, matching two K dwarfs. H1 (opaque disk) and H4 (dust) are not needed. Nothing anomalous remains.
C17 [observed] Referee: the data are not artifacts. Both ZTF fields (537 and 1532) show the eclipse. The in-eclipse ZTF centroid differs from out of eclipse by under 0.02 arcsec (IRSA per-epoch RA/Dec). Gaia G caught it on two cycles, including both FOV transits of one cycle (0.87 then 0.19 mag 106 min later). Gaia's resolution rules out a blend beyond about 0.5 arcsec, and no catalogued variable lies within 30 arcsec.
C18 [observed] Referee novelty search (2026-10-27): not catalogued as an eclipsing binary in VSX (only Type VAR, P=0.025 d from the Gaia short-timescale pipeline), SIMBAD (bare Gaia entry), ASAS-SN, ATLAS (J/AJ/156/241), ZTF Chen+2020, Gaia DR3 EB/classification/NSS, Prsa+2022 TESS EBs, or the Kostov+2025 TESS EB list (only in its unvetted input table). No literature found. Most likely an uncatalogued detached eccentric K+K eclipsing binary with P=11.9008 d: a modest catalogue addition, not an anomaly.
C19 [computed] The phase-clustering search alone found 2.38 days. The missing-eclipse test added after the referee's review (counting which cycles are deep versus flat, by cycle number mod k) flags at once that only every fifth cycle is deep: the true period is 5 x 2.38 = 11.90 days.
C20 [inferred] Explained: an uncatalogued, detached, mildly eccentric eclipsing binary of two K dwarfs, P = 11.900804 d (T0 = MJD 57099.3619, e about 0.15), with a primary eclipse of 0.9 mag and a secondary of about 0.4 mag at phase 0.404. Gaia's single-star parameters (R 1.07 Rsun, L 0.65 Lsun) describe the blended pair.

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