Default times, non arbitrage conditions and change of probability   measures
  In this paper we give a financial justification, based on non arbitrage conditions, of the $(H)$ hypothesis in default time modelling. We also show how the $(H)$ hypothesis is affected by an equivalent change of probability measure. The main technique used here is the theory of progressive enlargements of filtrations. 
Delia Coculescu, Monique Jeanblanc and Ashkan Nikeghbali