In this document (p.6 eqn. 2.1) and also in the general literature for the the basic Cooper pair box circuit the total capacitance \(C_\Sigma\) is the sum of the gate-capacitance \(C_g\) and the capacitance of the Josephson-Junction \(C_J\). Why is that the case? As far as I can see the elements are connected in series and thus only the inverses add up to the inverse of the total capacitance. What am I missing? Are the capacitances actually connected in parallel? If so can somebody explain why they are considered in parallel, because I don't understand it.

Thanks in advance!