We investigate observational signatures of static spherically symmetric black holes with a scalar field coupled to the Gauss-Bonnet term. For ₀₌ = -5 10^-3 and ₕ/M ₋ = 0. 020, full ray-tracing yields a photon sphere at r ₇ = 3. 010M and impact parameter b ₇ = 5. 071M, giving a shadow diameter deviation of only 0. 048\% from Schwarzschild. The dynamical properties are significantly altered: the Lyapunov exponent drops to M = 0. 112 versus 0. 1910 in General Relativity, increasing the instability timescale by 70. 5\%. For M87* (M = 6. 5 10⁹ M_), we predict = 249 25 hours, a ringdown delay of 104 hours relative to GR. Crucially, sub-ring flux ratios Iₙ/I₀ e^-n/ M are suppressed by factor >10⁵ for n 1, creating a "clean shadow" signature. We propose that ngEHT Phase-2 can test this model via light-curve autocorrelation of the n=0 ring: measuring / ₊₄ₑₑ = 1. 70 0. 10 favors this hairy solution, while detection of any sub-ring with I₁/I₀ > 10^-9 or / ₊₄ₑₑ = 1. 00 0. 05 rules it out.
Gustavo Rodrigues Dos Santos (Fri,) studied this question.