In the model described by the dynamics (1)–(2) under the physical measure, themarket
volatility σm is assumed to be constant. This is indeed not realistic both from the point
of view of the market returns distribution and the market skews of implied volatilities.
In this section, we propose to introduce stochastic volatility in the market model.
We will follow the approach taken in Fouque et al. (2000) and also used recently in
Fouque and Kollman (2009) in the CAPM context. In this generalized CAPM model,
the market volatility is driven by a fast mean-reverting factor according to