  We report on the stability of hypothetical Super-Earths in the habitable zone
of known multi-planetary systems. Most of them have not yet been studied in
detail concerning the existence of additional low-mass planets. The new N-body
code GENGA developed at the UZH allows us to perform numerous N-body
simulations in parallel on GPUs. With this numerical tool, we can study the
stability of orbits of hypothetical planets in the semi-major axis and
eccentricity parameter space in high resolution. Massless test particle
simulations give good predictions on the extension of the stable region and
show that HIP 14180 and HD 37124 do not provide stable orbits in the habitable
zone. Based on these simulations, we carry out simulations of 10 Earth mass
planets in several systems (HD 11964, HD 47186, HD 147018, HD 163607, HD
168443, HD 187123, HD 190360, HD 217107 and HIP 57274). They provide more exact
information about orbits at the location of mean motion resonances and at the
edges of the stability zones. Beside the stability of orbits, we study the
secular evolution of the planets to constrain probable locations of
hypothetical planets. Assuming that planetary systems are in general closely
packed, we find that apart from HD 168443, all of the systems can harbor 10
Earth mass planets in the habitable zone.
