Sonic-point conditions for our sample of putatively single WC/WO stars in the Galaxy and LMC. Density and temperature at the sonic point are expressed in terms of gas and radiation pressure P_gas and P_rad. Blue crosses indicate results from our empirical analysis in Sect. 4.1, black dots indicate estimates based on β-type wind models from Sect. 4.2. Both methods indicate a sonic-point relation with P_rad/P_gas = const. Uncertainties for the semi-empirically determined data points are discussed in Sect. 5.3. For the ratio of P_rad/P_gas they are likely of the order of 0.3 dex, but may be higher for individual positions along the indicated relations.