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The gravitational potential phi = GM/Rc^2 at the surface of the white dwarfG191-B2B is 10,000 times stronger than that at the Earth's surface. Numerousphotospheric absorption features are detected, making this a suitableenvironment to test theories in which the fundamental constants depend ongravity. We have measured the fine structure constant, alpha, at the whitedwarf surface, used a newly calibrated Hubble Space Telescope STIS spectrum ofG191-B2B, two new independent sets of laboratory Fe V wavelengths, and newatomic calculations of the sensitivity parameters that quantify Fe V wavelengthdependency on alpha. The two results obtained are: dalpha/alpha = 6.36 +/-[0.33(stat) + 1.94(sys)] X 10^{-5} and dalpha/alpha = 4.21 +/- [0.47(stat) +2.35(sys)] X 10^{-5}. The measurements hint that the fine structure constantincreases slightly in the presence of strong gravitational fields. Acomprehensive search for systematic errors is summarised, including possibleeffects from line misidentifications, line blending, stratification of thewhite dwarf atmosphere, the quadratic Zeeman effect and electric field effects,photospheric velocity flows, long-range wavelength distortions in the HSTspectrum, and variations in the relative Fe isotopic abundances. None fullyaccount for the observed deviation but the systematic uncertainties are heavilydominated by laboratory wavelength measurement precision.

Measuring the fine structure constant on a white dwarf surface; a detailed analysis of Fe V absorption in G191-B2B