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The reflectivity of the gold-coating on the XRT mirrors is a decreasing function of energy with a number of additional (minor) absorption edges. The energy dependence has been calculated on the basis of theoretical values for the optical constants of gold, and tested by ground calibration measurements in the PANTER facility. By comparing the experimental and theoretical results (taking into account the non-parallel beam testing geometry and the fact that only a few X-ray lines can be used) the final on-axis effective area was derived. A satisfactory agreement was reached by using the optical constants available in the literature.