9. Inside a cylinder with a base radius of 6, there are two spheres with radii also of 6, and the distance between their centers is 13. If a plane $\alpha$ is made to be tangent to these two spheres and intersects the cylindrical surface to form an ellipse, then the length of the major axis of this ellipse is $\qquad$
9. 13 As shown in the figure, a section is made through the centers of the two spheres $O_{1}, O_{2}$ and the point where the plane $\alpha$ is tangent to the spheres. $G H=13$ is the required length.