According to current plans, the SIM/NASA mission will be launched just after
the end of operations for the Gaia/ESA mission. This is a new situation which
enables long term astrometric projects that could not be achieved by either
mission alone. Using the well-known perspective acceleration effect on
astrometric measurements, the true heliocentric radial velocity of a nearby
star can be measured with great precision if the time baseline of the
astrometric measurements is long enough. Since white dwarfs are compact
objects, the gravitational redshift can be quite large (40-80 km/s), and is the
predominant source of any shift in wavelength. The mismatch of the true radial
velocity with the spectroscopic shift thus leads to a direct measure of the
Mass--Radius relation for such objects. Using available catalog information
about the known nearby white dwarfs, we estimate how many masses/gravitational
redshift measurements can be obtained with an accuracy better than 2%. Nearby
white dwarfs are relatively faint objects (10 < V < 15), which can be easily
observed by both missions. We also briefly discuss how the presence of a long
period planet can mask the astrometric signal of perspective acceleration.Comment: 3 pages, 2 Figures. Proceedings of the IAU Symposium 261 : Relativity
in Fundamental Astronomy. 27 April - 1 May 2009, Virginia Beach, VA, USA.
refereed and accepted versio