Let  $C_1$  be the circle of radius 1 with centre at the origin. Let  $C_2$  be the circle of radius r with centre at the point A(6, 1), where 1 < r < 4. Two distinct common tangents PQ and ST of  $C_1$ and  $C_2$ are drawn. The tangent PQ touches  $C_1$ at P and  $C_2$  at Q. The tangent ST touches $C_1$  at S and $C_2$ at T. Midpoints of the line segment PQ and ST are joined to form a line which meets the x–axis at point B. If  $AB=\sqrt{10}$ , then the value 
of  $r^2$  is _______