Determination of the PPN Parameter gamma with the HIPPARCOS Data

In the processing of the Hipparcos data, relativistic effects in the propagation of light, like the aberration and the bending of light rays, were introduced at an early level in the modelling. Thanks to the accumulation of very accurate measurements of star positions at various elongations from the Sun, it was possible to assess by how much the PPN parameter gamma deviates from its General Relativity value. A series of tests has been implemented to determine this parameter and evaluate the magnitude of the possible sources of errors. From the results we can conclude that the coefficient gamma is within 0.3 per cent of unity, with gamma = 0.997 +/- 0.003. This value is not significantly different from unity, as predicted by General Relativity.