Baade-Wesselink analysis of V439 Oph. Panels a, b, and d show our V, K_S, and radial velocity measurements, respectively. Panel c shows the unreddened colour index curve. Black lines in these plots are Akima splines fitted to the measurements. Panel e shows the integrated radial velocity curve. Panel f shows the angular diameter measured using the K04b SBCR. The green line in this panel is the curve from panel e, rescaled using equation 4 and measured values of the p-factor and the mean radius. Panel g shows the relation between angular diameters from panel f and corresponding values of the integrated radial velocity from panel e, rescaled using equation (4). The slope of this relation is the p-factor, and the zero point is the mean angular diameter used to calculate the mean radius of the star.