Empirical studies have concluded that stochastic volatility is an important
component of option prices. We introduce a regime-switching mechanism into
a continuous-time Capital Asset Pricing Model which naturally induces stochastic
volatility in the asset price. Under this Stressed-Beta model, the mechanism is relatively
simple: the slope coefficient—which measures asset returns relative to market
returns—switches between two values, depending on the market being above or below
a given level. After specifying the model, we use it to price European options on the
asset. Interestingly, these option prices are given explicitly as integrals with respect to
known densities. We find that the model is able to produce a volatility skew, which is
a prominent feature in option markets. This opens the possibility of forward-looking
calibration of the slope coefficients, using option data, as illustrated in the paper.