Before revision:  This allows us to completely characterize the models which prevent arbitrage opportunities . In this respect, we prove two results on the martingale property of stochastic exponentials.
After revision:  This allows us to completely characterize the models which prevent arbitrage opportunities : this boils down to finding a density between a risk-neutral measure \mathbb{Q this respect, we prove two results on the martingale property of stochastic exponentials.