  One of the parameters fitted by Doppler radial velocity measurements of
extrasolar planetary systems is omega, the argument of pericenter of a given
planet's orbit referenced to the plane of the sky. Curiously, the omega's of
the outer two planets orbiting Upsilon Andromedae are presently nearly
identical: Delta-omega = omega_D - omega_C = 4.8 deg +/- 4.8 deg (1 sigma).
This observation is least surprising if planets C and D occupy orbits that are
seen close to edge-on (sin i_C, sin i_D > 0.5) and whose mutual inclination
Theta does not exceed 20 deg. In this case, planets C and D inhabit a secular
resonance in which Delta-omega librates about 0 deg with an amplitude of 30 deg
and a period of 4000 yr. The resonant configuration spends about one-third of
its time with |Delta-omega| < 10 deg. If Theta > 40 deg, either Delta-omega
circulates or the system is unstable. This instability is driven by the Kozai
mechanism which couples the eccentricity of planet C to Theta to drive the
former quantity to values approaching unity. Our expectation that Theta < 20
deg suggests that planets C and D formed in a flattened, circumstellar disk,
and may be tested by upcoming astrometric measurements with the FAME satellite.
