The Newtonian equations of motion for two-point masses $m$ and $m^{\prime}$ are derived from the following Lagrangian:
$$L = \frac{1}{2}m\mathbf{v}^{2} + \frac{1}{2}m^{\prime}\mathbf{v}^{\prime \, 2} + \frac{Gmm^{\prime}}{R} \, ,$$ where $R = |r - r^{\prime}|$. In Damour, T.; Deruelle, N. (1985), “General relativistic celestial mechanics of binary systems. I. The post-Newtonian motion”, it's stated the center-of-mass integral is:
$$\mathbf{K} = m \mathbf{r} + m^{\prime} \mathbf{r}^{\prime} - t \mathbf{P}_{N} \, .$$ Where does the term $t\mathbf{P}_{N}$ come from?