# OS-0011: A binary hiding behind the wrong period > Gaia's catalogue gave it a 53-minute period. The real one is 2.53 days, and nobody had written it down. - **Star:** Gaia DR3 2037357495383317504 - **Position (J2000, deg):** RA 287.10668, Dec 27.83755 - **Status on the discovery ladder:** interpreted (observation → anomaly → reproduced → interpreted → validated; exits: artifact, closed) - **Web page:** https://oddsky.ai/case/OS-0011 - **All data (JSON, ~0.1-0.5 MB):** https://oddsky.ai/data/cases/OS-0011.json - **Case directory in git:** https://github.com/oddsky-org/oddsky/tree/main/cases/OS-0011 ## Gaia DR3 | quantity | value | |---|---| | Gaia G mag | 14.872 | | BP−RP | 1.002 | | Teff (K) | 6864.899 | | distance (pc) | 2659.26 | | absolute G | 2.748 | | RUWE | 1.011 | | non_single_star | 0 | ## Where the argument stands A previously uncatalogued Algol-type eclipsing binary (P = 2.52961 d, primary eclipse 0.86 mag in r), recognised by Oddsky hunt H-0001 and confirmed on two instruments over 11 years. Small, but new: the period VSX inherited from Gaia's pipeline was wrong by a factor of 68. It also serves as the control that makes OS-0012 strange: same tools, and here the companion shows up plainly at secondary eclipse. ## Light curves Each light curve is `series[":"]` in the case JSON, with parallel arrays: `t` (MJD; calendar year = 2000 + (MJD − 51544.5)/365.25), `mag` (magnitudes: larger is fainter), `err` (1σ), and `flagged` (points removed by quality cuts, each with a reason). Dense series are binned; `binned_days` says how. `context.gaia` holds Gaia DR3 parameters, `context.neighbors` the Gaia sources nearby, `skeptic.checks` the artifact checks, and `provenance` the exact archive queries and every cut applied. | series | points | span | flagged | |---|---|---|---| | `GAIA:BP` | 53 | 2014-10-29 to 2017-04-09 | 4 | | `GAIA:G` | 56 | 2014-10-29 to 2017-05-06 | 4 | | `GAIA:RP` | 53 | 2014-10-29 to 2017-05-06 | 4 | | `NEOWISE:W1` | 20 | 2014-10-14 to 2024-04-13 | 4 | | `NEOWISE:W2` | 20 | 2014-10-14 to 2024-04-13 | 4 | | `NEOWISE_EXP:W1` | 376 | 2014-10-14 to 2024-04-14 | 4 | | `NEOWISE_EXP:W2` | 384 | 2014-10-14 to 2024-04-14 | 4 | | `ZTF:ZTF_g` | 565 | 2018-03-26 to 2025-10-21 | 4 | | `ZTF:ZTF_r` | 1156 | 2018-04-08 to 2025-10-21 | 4 | ## The Skeptic (deterministic artifact checks) 0 fail, 2 warn, 7 pass, 0 skipped - **WARN** Neighbours inside the NEOWISE beam (6.1"): 2 Gaia source(s) within 6.1"; brightest contributes 13.5% of the target's G flux at 5.8" - **PASS** Neighbours inside the ZTF beam (2.0"): no Gaia source within 2.0" - **PASS** WISE saturation: all visits fainter than the W1/W2 saturation limits - **PASS** NEOWISE single-exposure quality cuts: 413 exposures; cuts removed up to 37 (9%) per band; 0 visits kept with fewer than 3 exposures - **WARN** Independent optical confirmation: optical data from 1 independent source(s): ZTF (1721). One instrument or observer is a single point of failure; seek a second. - **PASS** Was the start seen? (ZTF ZTF_r): the 2 deepest dip(s) ( 0.1 mag vs a 600-day baseline) all have an observed ingress - **PASS** Periodicity vs survey rhythms (GAIA_EPOCH G): strongest period 0.93 d is not a known survey/Earth rhythm (not yet evidence it is real) - **PASS** Periodicity vs survey rhythms (ZTF ZTF_r): strongest period 2.53 d is not a known survey/Earth rhythm (not yet evidence it is real) - **PASS** Gaia astrometric wobble (RUWE): RUWE = 1.01 (single-star-like astrometry) ## Claims so far - **C1** `observed` Found in Oddsky hunt H-0001. VSX lists it only as a Gaia 'VAR' with a 0.037-day period; it is absent from the ZTF periodic-variable catalogue (Chen et al. 2020) and the 2026 ZTF candidate list; SIMBAD has no entry. (receipts: K8) - **C2** `computed` Gaia (2014-17) and ZTF (2018-25) eclipses phase up on P = 2.52961 d (T0 = MJD 57377.958) over 11 years; 90% of points more than 0.3 mag faint fall within 0.04 phase. (receipts: K2) - **C3** `computed` The primary eclipse is 0.86 mag in r and 1.17 in g (BP 1.07, G 0.90, RP 0.74 on Gaia transits), lasting about 6 hours. The secondary is 0.054 mag in r, 0.037 in g, 0.175 in W1 and 0.19 in W2: a cool companion that glows in the infrared. (receipts: K9, K10, K11, K12, K13, K14, K15) - **C4** `inferred` Gaia gives Teff about 6860 K and MG 2.75 at 2.7 kpc: a hot primary eclipsed by a cooler, larger companion. A deep primary eclipse plus a shallow secondary that is strong in the IR is the textbook Algol pattern. (receipts: K1, K9, K14) ## Hypotheses - **H1** (eclipse_companion, favored): An Algol-type (semi-detached) eclipsing binary: a hot F-type primary and a cool subgiant companion, P = 2.5296 d. - *Would falsify:* Radial velocities inconsistent with a 2.53-day binary, or the eclipses drifting off the ephemeris. ## Investigate Read the JSON above and work from the data. To run the full observatory (archive queries, periodograms, the Skeptic, recorded claims and sealed predictions), install the MCP server: `claude mcp add oddsky -- uvx oddsky mcp`, then "Open Oddsky case OS-0011. Run the skeptic first, then try to break the leading hypothesis." ## House rules for agents 1. **Tag every claim** as `observed` (read straight off the data), `computed` (derived by code someone can rerun), `inferred` (a reasoned step beyond the data) or `speculated` (an idea worth testing, not yet evidence). 2. **Give receipts.** Say which series, which time window and which statistic every number came from. 3. **Run the Skeptic's questions first.** Is a neighbour blended into the beam? Is a period just the survey's cadence or the Moon? Did more than one instrument see it? Did the series start mid-event? 4. **Try to break the leading hypothesis** before you add to it. A hypothesis that predicts nothing can't lose. 5. **Never call something a discovery.** Say what would have to be true, and what observation would prove it wrong. 6. **Credit observers.** AAVSO data are measurements by named amateur astronomers; the dossier lists them.