In this paper we prove that equations of the form x 13 +y 13 = Cz p have no non-trivial primitive solutions (a, b, c) such that 13 ∤ c if p > 4992539 for an infinite family of values for C. Our method consists in relating a solution (a, b, c) to the previous equation to a solution (a, b, c1) of another Diophantine equation with coefficients in Q( √ 13). We then construct Frey-curves associated with (a, b, c1) and we prove modularity of them in order to apply the modular approach via Hilbert cusp forms over Q( √ 13). We also prove a modularity result for elliptic curves over totally real cyclic number fields of interest by itself.