Helioseismic determination of the solar gravitational quadrupole moment
  One of the most well-known tests of General Relativity (GR) results from combining measurements of the anomalous precession of the orbit of Mercury with a determination of the gravitational quadrupole moment of the Sun J_2. The latter can be done by inference from an integral relation between J_2 and the solar internal rotation. New observational data of high quality obtained from the Solar Heliospheric Satellite (SoHO) and from the Global Oscillations Network Group (GONG), allow the determination of the internal rotation velocity of the Sun as a function of radius and latitude with unprecedented spatial resolution and accuracy. As a consequence, a number of global properties of the Sun can also be determined with much higher accuracy, notably the gravitational quadrupole moment of the Sun. The anomalous precession of the orbit of Mercury is primarily due to GR effects but there are classical corrections the largest of which is that due to J_2. It is shown here that the data are currently consistent with the predictions of GR. 
F.P. Pijpers