Let $C_1$  and  $C_2$ are two circles whose equations are $x^2+y^2=r^2$  and ${(x-r)}^2+y^2=r^2$  respectively. If AB is their common chord and a straight line through A meets the circle $C_1$  at D and circle $C_2$  at E, then the $\angle DBE$  is