  Five planets are known to orbit the star 55 Cancri. The recently-discovered
planet f at 0.78 AU (Fischer et al. 2008) is located at the inner edge of a
previously-identified stable zone that separates the three close-in planets
from planet d at 5.9 AU. Here we map the stability of the orbital space between
planets f and d using a suite of n-body integrations that include an
additional, yet-to-be-discovered planet g with a radial velocity amplitude of 5
m/s (planet mass = 0.5-1.2 Saturn masses). We find a large stable zone
extending from 0.9 to 3.8 AU at eccentricities below 0.4. For each system we
quantify the probability of detecting planets b-f on their current orbits given
perturbations from hypothetical planet g, in order to further constrain the
mass and orbit of an additional planet. We find that large perturbations are
associated with specific mean motion resonances (MMRs) with planets f and d. We
show that two MMRs, 3f:1g (the 1:3 MMR between planets g and f) and 4g:1d
cannot contain a planet g. The 2f:1g MMR is unlikely to contain a planet more
massive than about 20 Earth masses. The 3g:1d and 5g:2d MMRs could contain a
resonant planet but the resonant location is strongly confined. The 3f:2g,
2g:1d and 3g:2d MMRs exert a stabilizing influence and could contain a resonant
planet. Furthermore, we show that the stable zone may in fact contain 2-3
additional planets, if they are ~50 Earth masses each. Finally, we show that
any planets exterior to planet d must reside beyond 10 AU.
