Illustration of the fit results for f_SP. We show the key figures for S2. Two left panels: the residuals in RA (top) and Dec (bottom) between the GRAVITY (cyan filled circles, and 1σ uncertainties) and NACO data (blue filled circles) and the best GR fit (red curve, f_SP = 1, Rømer effect, plus special relativity, plus gravitational redshift and Schwarzschild precession), relative to the same orbit for f_SP = 0 (Kepler/Newton, plus Rømer effect, plus special relativity, plus redshift). The orbital elements for non-Keplerian orbits (i.e., with f_SP ≠ 0) are interpreted as osculating parameters at apocenter time, 2010.35. NACO and GRAVITY data are averages of several epochs. The grey bars denote averages over all NACO data near apocenter (2004–2013). Top right: the same for the residual orbital angle on the sky δϕ = ϕ(f_SP = 1)−ϕ(f_SP = 0). Bottom right: zoom into the 2005/2021 part of the orbit, plotted in the mass rest frame. The earlier orbital trace does not coincide with the current one due to the Schwarzschild precession.