Power expansion in terms of a fraction of two variables

I have the following complicated expression:

(3 (Sqrt(6) a^13 + a^12 e0 + Sqrt(6) a^11 e0^2 + a^10 e0^3 + Sqrt(6) a^9 e0^4 - a^8 e0^5 -Sqrt(6) a^7 e0^6 - a^6 e0^7 + Sqrt(6) a^5 e0^8 - a^4 e0^9 - Sqrt(6) a^3 e0^10 + a^2 e0^11 -Sqrt(6) a e0^12 + e0^13))/(256 e0 (a^6 + Sqrt(6) a^5 e0 + a^4 e0^2 + Sqrt(6) a^3 e0^3 - a^2 e0^4 + Sqrt(6) a e0^5 - e0^6)^2)

The numbers are huge and not very important. In fact, I am only interested in the behavior of the expression in the limit $a/e_0 to 0$, so I would like to write it as a series of the form

constant + A (a/e_0) + O((a/e_0)^2),

for some coefficient A.
How can I do that automatically with Mathematica? I tried to use the Series function, but it cannot to an expansion for the variable $a/e_0$.