Before revision:  One of the key features of our model is that we specify the factor betas as functions of time-varying observed instruments that pick up long-run beta fluctuations, plus an orthogonal idiosyncratic component that captures high-frequency movements in beta .
After revision:  One of the key features of our model is that we specify the factor betas as the sum of characteristic effects and an orthogonal idiosyncratic component that captures high-frequency movements in beta .