XRP 15-Minute Polymarket Up/Down: Late-Window Favourite Underpricing, Confirmed by a Binance-Perp Fair-Probability Model (buy the DOWN token at 0.70-0.93 when the model says it is worth >=5 points more, hold to settlement)

Original Research Lead hypothesis, prediction-market archetypes 2+1 combined (calibration / favourite-longshot bias, with a crypto-data fair-value check). Tradeable unit: the recurring POLYMARKET series xrp-updown-15m, outcome token DOWN (instrument family XRP-UPDOWN-15M-<ts>-DOWN.POLYMARKET, 1-MINUTE probability bars, via PolymarketSeriesStrategy + a polymarket_series spec {series: 'xrp-updown-15m', outcome: 'Down', lookback_days: 70}). Signal source: XRPUSDT.BINANCE USD-M perp 1-MINUTE bars as an extra instrument (never traded). Each market resolves DOWN=1 if XRP's reference price at the window close is below its price at the window open. Mechanism: in the second half of a 15-minute window, once XRP has already moved down, the remaining time is short enough that the probability of a sign reversal is computable from the move size and short-horizon realised volatility. Retail binary books are documented to under-price favourites and over-price longshots (favourite-longshot bias), and the up/down books are thin, so the favourite token often lags the arithmetic. We buy the DOWN token only when BOTH (a) its price sits in the favourite band 0.70-0.93 and (b) a frozen Gaussian model built from Binance XRP 1-minute data says its fair probability is at least edge_min (5 points) above the market price. Hold to settlement (settlement is fee-free, so only one taker fee is paid). Why XRP and the DOWN series: SOL-15m and ETH-4h fair-value-lag hypotheses already in the pipeline buy the UP token mid-window; this one tests a different element (late-window favourites only, either-price-band gated) on an unused coin and on the complement token, so the factory gets an independent read of the calibration bias. The template trades one outcome token per series spec; the UP series is the natural breadth replication if this survives. Pre-study disclosed (not used to accept or reject): on a random 250-market sample, UNCONDITIONALLY buying the first DOWN price in 0.75-0.93 during 50-75% of the window won 87.2% at an average price 0.844 (net about +1.4% per dollar after fee and 1% spread); the same rule on the UP token won 79.1% at 0.840 (about -8.2%). So the raw favourite bias is not reliable by itself; the hypothesis is that the model-confirmation filter isolates the underpriced subset. History: the catalogue holds ~7,000 resolved xrp-updown-15m markets since 2026-08-13 (~76 days); lookback_days 70 is fillable. Execution: Polymarket is a CASH long-only venue; 'betting up' would be the UP series, never a short. Orders are market (taker) orders at the decision bar's close, which on this continuous 24/7 market stands in for the next bar's open (registered substitution for next_bar_open_fill). No stop: the maximum loss of each position is its premium, which IS the risk budget (route (b): time/settlement exit with capped sizing). Basis risk is named: Polymarket resolves on the Chainlink XRP/USD data stream, our model uses the Binance XRPUSDT perp; the edge_min margin is meant to absorb that difference.