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We carry out first-principles calculations for CaTi/BaTisuperlattices with epitaxial strain corresponding to growth on a SrTi substrate, and consider octahedral rotations as well as ferroelectric distortions. The calculations are done as a function of electric displacement field, and both a macroscopic and a local electrostatic analysis are carried out. We find that strong octahedral rotations occur for Ti octahedra sandwiched between CaO layers on both sides, but are strongly suppressed if either neighboring layer is a BaO layer. Due to the resulting enhancement of the ferroelectric instability in these octahedra, we find that overall the ferroelectric instability of the superlattice is enhanced by the interface. Thus, short-period superlattices in this system have a larger ferroelectric polarization than longer-period ones of the same average composition, contrary to the expected trend.