... Daigne

Present address: Service d'Astrophysique, CEA/Saclay, 91191 Gif-sur-Yvette Cedex, France.

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... factor

For commodity, we use the expression "Lorentz factor'' for  $\tilde{u}^{\rm t}$ even if, strictly speaking, this should only be used at large distance of the source where the metric is very close to the Minkowski metric, i.e. for $x \gg 1$. The correct expression of the Lorentz factor should be corrected with the lapse function, $\Gamma = \alpha \tilde{u}^{\rm t}$, where $\alpha=1/\sqrt{-\tilde{g}^{tt}} \to 1$ for $x \to +\infty$.

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