Recent results have shown that many of the known extrasolar planetary systems contain regions which are stable for massless test particles .
We examine the possibility that Saturn-mass planets exist in these systems , just below the detection threshold , and attempt to predict likely orbital parameters for such unseen planets .
To do this , we insert a Saturn-mass planet into the stable regions of these systems and integrate its orbit for 100 million years .
We conduct 200-600 of these experiments to test parameter space in HD37124 , HD38529 , 55Cnc , and HD74156 .
In HD37124 the global maximum of the survival rate of Saturns in parameter space is at semimajor axis a = 1.03 AU , eccentricity e \sim 0.1 .
In HD38529 , only 5 % of Saturns are unstable , and the region in which a Saturn could survive is very broad , centered on 0.5 < a < 0.6 , e < 0.2 .
In 55Cnc we find three maxima at ( a , e ) = ( 1.0 AU , 0.02 ) , ( 2.0 AU , 0.08 ) , and ( 3.0 AU , 0.17 ) .
In HD74156 we find a broad maximum with a = 0.9-1.2 AU , e \leq 
0.15 .
Several of these maxima are located in the habitable zones of their parent stars and are therefore of astrobiological interest .
We suggest the possibility that companions may lie in these locations of parameter space , and encourage further observational investigation of these systems .