Closed-form convexity and cross-convexity adjustments for Heston prices

We present a new and general technique for obtaining closed-form expansions for prices of options in the Heston model, in terms of Black-Scholes prices and Black-Scholes Greeks up to arbitrary order. We then apply the technique to solve, in detail, the cases for the second-order and third-order expansions. In particular, such expansions show how the convexity in volatility, measured by the Black-Scholes volga, and the sensitivity of delta with respect to volatility, measured by the Black-Scholes vanna, impact option prices in the Heston model. The general method for obtaining the expansion rests on the construction of a set of new probability measures, equivalent to the original pricing measure, and which retain the affine structure of the Heston volatility diffusion. Finally, we extend the method to the pricing of forward-starting options in the Heston model.