Let J 65 be the Jacobian of the Shimura curve attached to the indefinite quaternion algebra over Q of discriminant 65. We study the isogenies J 0 (65) → J 65 defined over Q, whose existence was proved by Ribet. We prove that there is an isogeny whose kernel is supported on the Eisenstein maximal ideals of the Hecke algebra acting on J 0 (65), and moreover the odd part of the kernel is generated by a cuspidal divisor of order 7, as is predicted by a conjecture of Ogg.2010 Mathematics Subject Classification. 11G18. 1 2 KRZYSZTOF KLOSIN AND MIHRAN PAPIKIAN the isogenies between them. Ogg's conjecture remains open except for the special cases when J N has dimension ≤ 3.When dim(J N ) = 1, equiv. N = 2 · 7, 3 · 5, 3 · 7, 3 · 11, 2 · 17, J N is an elliptic curve over Q which is uniquely determined by its component groups at p and q, and J 0 (N ) new is the optimal elliptic curve of conductor N . Then one easily checks Ogg's conjecture using Cremona's tables [5]. In general, the orders of component groups of J N can be computed using Brandt matrices [10], which is relatively easy to do with the help of a computer program such as Magma.When dim(J N ) = 2, equiv. N = 2 · 13, 2 · 19, 2 · 29, Ogg's conjecture is verified in [7]. In this case, the proof is based on the fact that X N is bielliptic and the lattices of J 0 (N ) new and J N can be computed through their elliptic quotients.When dim(J N ) = 3, equiv. N = 2 · 31, 2 · 41, 2 · 47, 3 · 13, 3 · 17, 3 · 19, 3 · 23, 5 · 7, 5 · 11, Ogg's conjecture is verified in [6]. In this case, X N is always hyperelliptic. By utilizing this fact, González and Molina explicitly compute the equation for each X N . Then they obtain a basis of regular differentials for X N from these equations to produce a period matrix for J N . The period matrix of J 0 (N ) new can be computed using cusp forms with rational q-expansions. The problem then reduces to comparing the period matrices of appropriate quotients of J 0 (N ) new with the period matrix of J N .The goal of this paper is to study Ribet's isogeny for N = 5 · 13 = 65. In this case, dim(J N ) = 5 and X N is not hyperelliptic; cf. [14]. Our approach to the study of Ribet isogenies is completely different from that in [7] and [6], and crucially relies on the Hecke equivariance of such isogenies. In this approach we need to know very little about X N or J N ; we only need to know the orders of component groups of J N , which, as we mentioned, are easy to compute, and in fact were already computed in [16]. The difficulty shifts to the study of the structure of the Hecke algebra and its action on J 0 (N ).Let T(N ) := Z[T 2 , T 3 , . . . ] be the Z-algebra generated by the Hecke operators T n acting on be the space S 2 (N ) of weight 2 cups forms on Γ 0 (N ). This algebra is isomorphic to the subalgebra of End(J 0 (N )) generated by T n acting as correspondences on X 0 (N ). When N = 65, we have J 0 (N ) new = J 0 (N ), so there is a Ribet isogeny π : J 0 (N ) → J N . T(N ) also naturally acts on J N and π is T(N )-equivariant. This equivari...