What is the relationship between Helium's atomic mass and the Rydberg constant?

[MaestroBach]

Homework Statement

Determine the ratio of the Rydberg constant for Helium to Hydrogen

Relevant Equations

R = (Z^2*m*e^4)/(8*E0*h^3 * c)

I thought the ratio was purely 4, because Helium's Z = 2 (atomic mass). Apparently this is not true, and now I'm stuck... I think there must be some other form of equation for the rydberg constant that is more general, but google and wikipedia are not helping that much.

Any thoughts?

 

[TSny]

Hint: "reduced mass"