A dust-shrouded star in a star-forming region goes dark for a week every 385.7 days

Found in Oddsky hunt H-0004 while looking for R CrB declines. Tisserand+2020 WISE candidate 2682 (2MASS J18205507-1340452), 4 arcsec from IRAS 18180-1342: steady to 0.1-0.2 mag, then four sharp 1.5-2.2 mag eclipses in four consecutive years, deeper in blue, on a strict 385.7-day period. Listed as a 'dubious' ATLAS variable and a Gaia DR2 long-period-variable candidate; the eclipses themselves were never reported.

Four full-depth eclipses (2019-2022) on P = 385.7 +/- 0.5 d, T0 = MJD 58702.2, O-C under 0.7 d; depths 1.5-1.65 mag in r and 1.8-2.2 in g, varying by 0.2-0.4 mag between orbits; no secondary deeper than 0.1 mag near phase 0.5. The star is at 2.4 kpc with L ~ 20-100 Lsun (R under ~10 Rsun), a Class II mid-infrared SED and RUWE 1.8, in or behind an intermediate-mass star-forming region. The occulter is a translucent, reddening, evolving dust structure on a one-year orbit, whether around a companion of an evolved star or in the disk of a young binary. Referee (2026-10-27): the eclipses survive; mundane explanations about 5%. Predictions sealed for the 2027 quiet season and the 2028 January eclipse; ATLAS (independent telescopes and pipeline) shows the same eclipses on the same ephemeris, including the 2023 one ZTF missed: reproduced.

Claims

C1 [computed] ZTF r is flat at 14.77 mag (robust scatter ~0.02 mag) for 7.5 years apart from brief deep fades.
C2 [observed] Three complete eclipses were observed: MJD 58692-58715 (r minimum 16.41 on 58701.2, depth 1.65 mag, 9-day ingress, egress complete within 14 days), MJD 59466-59488 (g minimum 18.07 on 59470.2, depth 2.1 mag in g, 1.5 in r), MJD 59853-59866 (r 16.2-16.27 on 59856-59860, g 17.83 on 59858). In-eclipse nights carry 5-7 exposures agreeing to 0.01-0.02 mag.
C3 [observed] A fourth event is caught in ingress: r fades from 14.79 (MJD 60231) to 14.99-15.07 on MJD 60251-60254, then the observing season ends.
C4 [computed] Phase-clustering of all points fainter than 0.5 mag below median gives P = 386.96 d, T0 = MJD 58700.1, with 100% of deep points within 0.04 in phase and 0.3% of quiet points in the eclipse core; the missing-cycle test finds no evidence for a multiple of this period. Timing of the three minima (58701, 59471, 59858) refines this to P = 385.7 +/- 1 d.
C5 [computed] The eclipse is deeper in g than in r: folded depths None (g) and None (r) mag; the g-r colour slope against r brightness is 1.3047697789596673 (ISM-dust expectation None). The occulter reddens the star: dust, with grains that are not much greyer than interstellar ones.
C6 [computed] No secondary eclipse deeper than the folded noise is seen at phase 0.5 in r or g.
C7 [observed] Gaia DR3: G = 14.76, BP-RP = 2.01, parallax 0.417 +/- 0.045 mas (2.4 kpc), RUWE = 1.81, at l = 17.3, b = +0.4. Nearest Gaia neighbours: G 18.5 at 3.0 arcsec (3% of the flux) and G 16.1 at 8.2 arcsec (29%, outside the ZTF aperture). WISE W1-W4 = 7.95, 7.17, 4.21, 2.90: a large mid-infrared excess. 2MASS J-K = 2.1.
C8 [computed] Lomb-Scargle on the r series finds no significant periodicity apart from aliases; the eclipses are too brief (about 4% of the period) for a sinusoid search, which is why Gaia DR3 and ZTF classifiers did not flag it. VSX has no entry at this position. SIMBAD lists it as an intrinsically red Spitzer source (Robitaille+2008), a SPICY candidate young star (Kuhn+2021), a Gaia DR2 long-period-variable candidate and a WISE-colour R CrB candidate (Tisserand+2012, 2020; Karambelkar+2024 priorities).
C9 [inferred] With P = 386 d and a total mass of 1-3 Msun the orbit is 1.1-1.6 AU. An eclipse lasting 15-20 days covers 4-5% of the orbit, so the occulter is roughly 60-100 Rsun across: far larger than any star, the size of a dusty disk around a companion.
C10 [computed] Folded at P = 385.7 d, T0 = MJD 58701.0: the deepest in-eclipse point is 1.66 mag below the 14.76 baseline in r (K28) and 2.19 mag below the 15.99 baseline in g (K29); at phase 0.5 the deepest point is 0.09 (r) and 0.14 (g) mag, i.e. no secondary eclipse above the 0.03-0.04 mag out-of-eclipse scatter. Even and odd cycles both reach full depth (r: 1.66 and 1.58 mag), so the period is not 2 x 385.7 d. Simultaneous g and r (253 pairs within a day) give d(g)/d(r) = 1.30 with r = 0.99 (K15): the occulter reddens the star like dust with grains somewhat larger than interstellar ones (ISM ratio ~1.4, grey = 1.0).
C11 [computed] Period and aliases survive. eclipseperiod on r+g (depth 0.5) and r alone (depth 1.0) returns 387.0 and 387.4 d with every deep point within 0.012 in phase (K41, K42). Folds at P/2 = 192.85 d, 365.25 d and 182.6 d leave the odd cycles flat (median delta 0.06, -0.03, 0.04 mag against 1.40, 0.43, 1.37 for the even ones: K44, K47, K48); 2P and 3P put full-depth events at phase 0.51 and 0.67 (K45, K46), so the period is not a multiple. Four consecutive cycles show the full event (K43, K49). A common-template fit (pandas/scipy, outside the tools) to the r data of cycles 0-3 gives mid-times MJD 58701.6, 59088.6, 59473.9, 59858.8, a linear ephemeris P = 385.69 d, T0 = 58702.2, O-C within +/-0.7 d, FWHM 7.9 d, width 14 d at 0.05 mag. Cycles -1 (July 2018), 5 (Nov 2024) and 6 (Dec 2025) have no ZTF data within 8 d of the prediction (K62, K63), so they test nothing.
C12 [observed] Claim C2 undercounts the events: a fourth full-depth eclipse was recorded in cycle 1 (August 2020) on the ephemeris: g = 18.18 on MJD 59086.2 (2.19 mag below baseline), r = 16.34 on 59087.1 (1.57 mag), i = 14.60 on 59084.2 (0.59 mag) (K58, K59, K60); the catflags-rejected epochs add r = 15.86 on 59085.2 and i = 15.43 on 59086.3 (IRSA raw, outside the tools). Four complete-depth events in four consecutive cycles, not three.
C13 [computed] Claim C3 ('a fourth event caught in ingress') is not supported by the ephemeris. The 2023 points (MJD 60251-60254, 0.22 to 0.29 mag below baseline, still fading: K61) fall 7.3-10.3 d AFTER the predicted minimum 60245.0, where cycles 0, 2 and 3 were at 0.07-0.28 mag and recovering (K64, K65, K66). There is no ZTF point between dt = -12.7 and +7.3 d, so the 2023 eclipse itself was never observed. Shifting the joint template by +15 d fits better than no shift (chi2 4744 vs 10490, n = 23; outside the tools) but neither is acceptable. The 2023 data are consistent with the trailing edge of an on-time eclipse with a longer or structured egress, or with an unrelated shallow dip; they are not an ingress.
C14 [computed] Instrument artifacts are excluded (IRSA per-epoch ZTF metadata and pandas, outside the tools). The 50 in-eclipse epochs with catflags = 0 have median airmass 1.61 (quiet 1.63), limiting magnitude 20.6 (20.5), chi 0.94 (0.77), sharp 0.03 (-0.01) and zero-point rms 0.022 (0.028); both ZTF programs record eclipses (program 2 in 2019-2020, program 1 in 2021-2022); the centroid moves 0.06 arcsec in eclipse; nightly scatter among 5-11 exposures is 0.008-0.015 mag in and out of eclipse. The catflags-rejected epochs (58698, 58702, 59085, 59086, 59470) lie on the same eclipse shape. All data come from field 333 CCD 15 except 11 points of field 1376 that fall out of eclipse (K62). On every eclipse night the 8.2-arcsec G = 16.1, 17-arcsec G = 15.2 and 17.9-arcsec G = 13.8 neighbours are constant to 0.01-0.02 mag, as they are on the 2023 nights. The events fall in Aug 2019, Aug 2020, Sep 2021, Oct 2022, i.e. they advance through the season by 20 d/yr, which no seasonal systematic does.
C15 [computed] Blends cannot produce the dips. Within the ZTF PSF the only Gaia source is the 3.0-arcsec G = 18.5 star (3.3 per cent of the G flux; K52), which UKIDSS GPS puts at K = 16.1 against K = 9.9 for the target (VizieR, outside the tools). UKIDSS also shows two K = 13.3-13.6 sources at 4.0-4.4 arcsec with no Gaia counterpart (embedded, optically invisible). Removing 78 per cent of the r flux (1.65 mag) requires the target itself to fade; neighbours can only dilute the depth by under 5 per cent.
C16 [computed] 'No secondary' holds where it was tested and nowhere else. At P = 385.7 d the phase-0.5 window (+/-7.7 d) contains 12 r and 9 g points with median 0.05 and 0.10 mag below baseline and maxima 0.09 and 0.14 (K43, K49). The faintest non-primary bin is 0.12 mag at phase 0.41 (K43): a slow 0.13 (r) / 0.20 (g) mag dimming over MJD 60750-60790 (K74, K75, K78) with d(g)/d(r) = 1.7 (K76), dust-coloured, at phase 0.33-0.41 of cycle 5. Phases 0.25-0.45 are sampled in only one cycle because the eclipse drifts through the observing season, so an eccentric-orbit secondary away from phase 0.5 is not excluded.
C17 [computed] The baseline is not 'flat at 0.02 mag' (C1). Out of eclipse, nightly medians span 15.90-16.23 in g and 14.70-14.91 in r (2-98 per cent ranges 0.22 and 0.17 mag; pandas, outside the tools), season medians drift by 0.09 (g) and 0.04 (r) mag (K53, K54), and out-of-eclipse g and r move together with d(g)/d(r) = 1.23 (K77). Gaia DR2 saw a 0.22-mag, nearly grey (BP 0.23, RP 0.19) range in 17 transits in 2014-2016 and flagged the star VARIABLE / LPV candidate; ATLAS 2015-2017 measured an rms of 0.064 mag. The robust per-night scatter is 0.027 (r) and 0.041 (g).
C18 [observed] Novelty (VizieR, SIMBAD, Gaia archive, outside the tools; K70). The star IS in two variable-candidate catalogues: ATLAS Heinze+2018 ATOID J275.2295-13.6792 at 0.23 arcsec (class 'dubious', 77 c + 66 o points, long-period fit 406.8 d with 0.15 mag amplitude) and Gaia DR2 LPV candidates (Mowlavi+2018; Mbol = 1.5 +/- 0.7). Neither reports eclipses. SIMBAD types 2MASS J18205507-1340452 as 'AB?' (AGB candidate; Robitaille+2008); SPICY 82128 is Class II (alpha = -0.76, EnvIII); Tisserand+2020 candidate 2682. IRAS 18180-1342, 4.3 arcsec away, is an intermediate-mass star-forming region (Arvidsson+2010; Lundquist+2014, 2015); two YSO candidates lie at 17-18 arcsec and the molecular cloud SDG G17.308+0.392 at 28 arcsec. Absent from VSX (nearest entry 46 arcsec), ASAS-SN variables, ZTF Chen+2020, Gaia DR3 vari tables (no classifier result, flag NOTAVAILABLE), Marton+2019. Gaia DR2 epoch photometry (17 transits, MJD 56915-57505) misses the predicted cycle -4 (57158) by 56 d and constrains nothing. 'Not previously catalogued as variable' is false in letter and true in substance: the periodic eclipses are unreported.
C19 [inferred] Physical self-consistency. For Mtot = 1, 2, 3 Msun: a = 1.04, 1.31, 1.50 AU and vrel = 29, 37, 42 km/s, so the 18-19 d first-to-last contact is a chord of 67, 85, 97 Rsun and the 7.9-d FWHM 29-41 Rsun; this chord is Rocculter + Rstar, not the occulter alone. The eclipse is translucent and chromatic (d(g)/d(r) = 1.30: K50; g/i = 1.5 on 59086), so the dust must cover the whole stellar disk with taur ~ 1.5, i.e. Rocc = Rstar: that only works if the primary is small. It is: at AV = 3-4 (BP-RP0 = 0.3-0.8), MG = +0.4 to -0.5, and the Gaia DR2 Mbol = 1.5 +/- 0.7 gives L ~ 20-100 Lsun, R <~ 10 Rsun for Teff = 4000 K. The 'red giant' of the title is unsupported: the SED (J-H 1.09, H-K 1.02, K-W1 1.6, W1-W2 0.78, W2-W3 3.0), the Class II SPICY classification and the star-forming environment favour a reddened intermediate-mass pre-main-sequence star (or at most a low-luminosity giant). RUWE 1.81 (astrometric excess noise 0.29 mas at 19.6 sigma) is what a 0.4-mas photocentre wobble on a 1-AU orbit at 2.4 kpc would give. Depths differ between cycles (g: 2.19 in 2020, = 2.08 in 2021, 1.84 in 2022; r: 1.64, 1.57, 1.45-1.78, 1.54: K49, K43), so the occulter's column changes by 0.2-0.4 mag from orbit to orbit: a clumpy dust structure, not a rigid opaque disk.
C20 [computed] NEOWISE W1 and W2 are 0.09-0.12 mag brighter within +/-60 d of the eclipse phase than at phase 0.55-0.75 (K71, K72), in both even and odd cycles. NEOWISE samples only two phases per year and W1 also fades secularly (0.024 mag/yr: K56), so this is a lead, not a result; no NEOWISE visit falls inside an eclipse (closest: 59098, dt = +11.5 d of cycle 1, W1 7.94, bright; raw IRSA exposures, outside the tools).
C21 [computed] Four full-depth eclipses in four consecutive cycles: 2019 (r 1.65 mag), 2020 (g 18.18 on MJD 59086, found by the referee), 2021 (g 2.1, r 1.5), 2022 (r 1.5, g 1.8, flat-bottomed and 0.35 mag shallower in g). Timing all four gives P = 385.7 +/- 0.5 d, T0 = MJD 58702.2, O-C under 0.7 d; the 2023 and 2024 eclipses fell in ZTF gaps. The depths differ by 0.2-0.4 mag between orbits, so the occulter is a clumpy, evolving dust structure rather than a rigid opaque disk.
C22 [computed] ATLAS forced photometry (2015-2026, 613 good exposures in a crowded field, independent telescopes and pipeline) shows the same eclipses on the same ephemeris: cycle 0 in cyan (17.24 at mid-eclipse, 1.9 mag deep), cycle 2 in orange (16.09 one day after mid-eclipse, 1.6 mag, recovered by +8 d), cycle 3 in orange sampled on eight nights from -4 to +7 d (deepest 16.09 at -2 d), and the 2023 eclipse (cycle 4) that fell in a ZTF gap, caught in ingress at -2 and -1 d (15.32, 15.85: at least 1.4 mag deep, on time). Cycles -1 and 1 have ATLAS points only at +4 to +6 d, 0.2-0.45 mag faint, consistent with the egress wing. ATLAS has no data within 25 d of the 2024 and 2025 eclipses (conjunction season). Phase-clustering of ZTF and ATLAS together keeps P = 386-387 d with all deep points at one phase.

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