A linear limb law gives exact speed degeneracy in a narrow-ribbon model of the known KIC 11084727 dip

We test what a restricted dust model can measure from the known Kepler dip of KIC 11084727. For a straight narrow ribbon, small optical depth, and a free linear limb-darkening coefficient, a change in impact parameter can be offset by changes in limb darkening, speed, tail length, and loss amplitude. The predicted flux is exactly unchanged at all times. Exposure averaging and gaps preserve this equality. Kepler release 25 quarter 11 fits give example central-chord speeds of 77 to 84 km/s for an assumed radius of 1.55 solar radii. Equivalent models at impact parameter 0.9 give 34 to 36 km/s. These are examples, not speed measurements. The general geometry problem is known. We supply an explicit exact transformation, numerical checks, and a reproducible application. Reference fits reach the limb coefficient limit, so they do not measure a stellar intensity law. A fixed atmosphere law, finite ribbon width, or other physical information can restrict the symmetry. No new object, dust identification, or independent telescope confirmation is reported. The package contains the raw input member, source hashes, proof, code, fit tables, integration checks, and a figure.

exocomet · Kepler · KIC 11084727 · limb darkening · identifiability · dust tail