  Based on the long-term radial-velocity surveys carried out with the McDonald
Observatory 2.7m Harlan J. Smith Telescope from 1988 to the present, we derive
upper limits to long-period giant planet companions for 31 nearby stars. Data
from three phases of the McDonald Observatory 2.7m planet-search program have
been merged together and for 17 objects, data from the pioneering
Canada-France-Hawaii Telescope (CFHT) radial-velocity program have also been
included in the companion-limits determination. For those 17 objects, the
baseline of observations is in excess of 23 years, enabling the detection or
exclusion of giant planets in orbits beyond 8 AU. We also consider the
possibility of eccentric orbits in our computations. At an orbital separation
of 5.2 AU, we can exclude on average planets of M sin i > 2.0+/-1.1 Mjup (e=0)
and M sin i > 4.0+/-2.8 Mjup (e=0.6) for 25 of the 31 stars in this survey.
However, we are not yet able to rule out "true Jupiters," i.e. planets of M sin
i ~ 1 Mjup in 5.2 AU orbits. These limits are of interest for the Space
Interferometry Mission (SIM), Terrestrial Planet Finder (TPF), and Darwin
missions which will search for terrestrial planets orbiting nearby stars, many
of which are included in this work.
