Although the NAC point, where the three phases : nematic, smectic A and smectic C coexist, can be seen as a Lifshitz point, we think that it is interesting to propose a Landau theory with two order parameters. We are able to explain easily that the N-Sm C transition is always of first order, as experimentally observed. The Sm A-Sm C transition can be either first order or second order depending whether the transition point is near or far from the NAC point. The diagram topology we found is in agreement with the experimental results