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\label{sect:model}  In this section we develop an analytical model for the interface between the stellar wind and the surrounding disk wind. The pressure of the disk wind collimates the stellar wind into a jet (see figure~\ref{fig:sketch} for a sketch). The two flows are separated by a contact discontinuity, whose exact position is given by pressure equilibrium between the outer, disk wind component and the inner, stellar wind component. We assume that the stellar wind is initially emitted radially. As it encounters the contact discontinuity, the velocity component perpendicular to the discontinuity is shocked. Thus, our model needs to distinguish three zones: (i) the cold pre-shock stellar wind, (ii) the hot post-shock stellar wind, and (iii) the disk wind. Our goal is to calculate the geometrical shape of the stellar wind shock, since this determines the velocity jump across the shock front and the temperature of the post-shock plasma.   The Rankine-Hugoniot jump conditions relate the density $\rho$, velocity $v$, and pressure $P$ on both sides of a shock. For ideal gases and non-oblique shocks the conservation of mass, momentum and energy across the shock can be written as follows \citep[][chap.~7, \S~15]{http://adsabs.harvard.edu/abs/1967pswh.book.....Z}, \citep[][chap.~7]{1967pswh.book.....Z},  where the state before the front of the shock front is marked by the index 0, that behind the shock by index 1: \begin{eqnarray}  \label{eqn:RH1}\rho_0 v_0 &=& \rho_1 v_1\\  \label{eqn:RH2}P_0+\rho_0 v_0^2 &=& P_1+\rho_1 v_1^2\\