Independent_Gaia_XP_and_Roche_lobe_evidence_for_an_sdB_M_dwarf_configuration_in_TIC_322390461

TIC 322390461 (UCAC4 782-037084; Gaia DR3 2219505890166498048) was previously flagged as an HW Vir-type candidate by Barlow et al. (2022) based on TESS light-curve morphology. However, no medium- or high-resolution spectroscopy exists for this source, preventing a classical atmospheric classification through Balmer or helium line fitting. This work presents an independent physical validation of the system using only Gaia DR3 photometry, parallax, XP spectroscopy, and TESS photometry, without relying on spectroscopic line profiles. The methodology is inverted relative to conventional approaches: rather than fitting observed photometry to stellar models, we systematically test which physical stellar configurations can simultaneously satisfy all geometric and dynamical constraints imposed by the data. Key results: The Gaia XP continuum is strongly blue, and the absolute magnitude (MG ≈ 5.4) is incompatible with a hot white dwarf by several magnitudes but fully consistent with canonical sdB luminosities. The short orbital period (P = 0.193443 d) implies a compact configuration in which no main-sequence primary can satisfy the Roche-lobe constraint, whereas an sdB primary (R ≈ 0.15–0.25 R⊙) fits comfortably within its Roche lobe. The secondary is consistent with a low-mass M dwarf with a moderate filling factor (f ≈ 0.68), naturally explaining the suppressed reflection effect (~0.09%) in the TESS light curve. The system is detached and evolving toward Roche-lobe contact on a timescale of order 10⁸ yr through angular-momentum loss driven by gravitational-wave radiation and magnetic braking. Among all tested configurations — main-sequence primaries, white-dwarf primaries, and sdB primaries — only the sdB+dM configuration satisfies every geometric and dynamical constraint. The convergence of the Gaia-based luminosity diagnostics and the independent Roche-geometry analysis provides a physically robust classification without requiring spectroscopic confirmation. The methodology demonstrated here may serve as a framework for classifying faint hot-subdwarf candidates that lack spectroscopic data.