We report on the depression of the superconducting critical temperature of ultra thin YBa2Cu3O7 (YBCO) layers, when their thickness is reduced in presence of La0.7Ca0.3MnO3 (LCMO) magnetic layers in [LCMO (15 u.c.)/ YBCO(N u.c.)] superlattices. The thickness of the manganite layer is kept at 15 unit cells and the YBCO thickness is varied between N=12 and N=1 unit cells. The structural analysis using x-ray diffraction and electron microscopy shows sharp interfaces with little structural disorder. While a critical temperature, Tc=85 K is found for 12 YBCO unit cells, superconductivity is completely suppressed for YBCO layer thickness below 3 unit cells. The possible interaction between superconductivity and magnetism is investigated.