A rotary feed-through for mounting a rotating substrate tube in a lathe and providing a flow of process gas into the tube, said feed-through including a process gas supply line for providing a process gas into said substrate tube, a rotatable holder arranged for receiving and holding said substrate tube for rotating said substrate tube with respect to said process gas supply line, a rotary union provided between said rotatable holder and said process gas supply line for rotatably connecting said rotatable holder to said process gas supply line, a stationary housing connected to said process gas supply line and to said rotatable holder, therewith forming a closed cavity surrounding said rotary union, wherein said stationary housing further includes an auxiliary gas supply line for providing said closed cavity with an auxiliary gas.