Before revision:  Indeed we propose a simple model of the price dynamics in which the return growth depends on three components: a ) a momentum component, naturally justified in terms of agents' belief that expected returns are higher in bullish markets than in bearish ones ; b ) a fundamental component proportional to the log earnings over price ratio at time zero  , from which the actual stock price may deviate as an effect of random external disturbances, and c ) a driving component ensuring the diffusive behaviour of stock prices.
After revision:  Indeed we propose a simple model of the price dynamics in which the return growth depends on three components: a ) a momentum component, naturally justified in terms of agents' belief that expected returns are higher in bullish markets than in bearish ones , ii ) a fundamental component proportional to the log earnings over price ratio at time zero , from which the actual stock price may deviate as an effect of random external disturbances, and c ) a driving component ensuring the diffusive behaviour of stock prices.