at University of Iowa Libraries/Serials Acquisitions on June 1, 2015 http://imrn.oxfordjournals.org/ Downloaded from 1016 Ettore Aldrovandi if Pic(X) denotes the group of isomorphism classes of line bundles on X equipped with a smooth Hermitian metric, it is shown that Pic(X) ∼ = H 2 D (X; 1). (1.1) The groups H • D (X; p) have cup products behaving in the standard way: H k D (X; p) ⊗ H l D (X; q) ∪ − − → H k+l D (X; p + q). (1.2) Thus, given the two metrized line bundles as above we can multiply their classes in Pic(X) using this product to obtain a class [L] ∪ [M] ∈ H 4 D (X; 2). If X is a curve, the latter group can be replaced by H 2 (X, R), which yields the sought-after pairing after evaluating the resulting class against the fundamental class of X. (See [1] and Section 4 below. We have neglected the Tate twists by 2π √ −1 here.) The number so obtained is the length of a generator of L, M associated to a specific choice of l and m, see Section 5.1 below. The case L = M = T X turns out to be quite interesting on its own and relevant to the hyperbolic geometry of X considered as a Riemann surface. A Hermitian fiber metric is now just a conformal metric on X. If the genus is greater than or equal to 2, we have shown in [1, Theorem 5.1] that the cup square [T X ] ∪ [T X ], supplemented by the area form, determines a functional whose extremum is precisely the hyperbolic metric of constant curvature equal to −1. In fact, we have shown it coincides with the Liouville functional studied in [16, 18, 19], in relation to Weil-Petersson geometry. Returning to the general case of the pairing L, M where L and M are not necessarily equal, it has been observed-also by Deligne [10]-that the metric on the pairing could be defined also when the fiber metrics on both L and M are allowed to be singular, provided the corresponding loci are disjoint. In a different direction, if X/ Z is an arithmetic surface such that X is the corresponding fiber at infinity, Kühn [14] has formulated a pairing (generalizing Deligne's one) for line bundles where the metric at the infinite places-the connected components of X-has logarithmic singularities at a finite set of points. (This is shown in the same work to be compatible with an earlier version by Bost, cf. [6], based on Green's currents.) This leads to the interesting question of whether pairings with singular metrics can be expressed in terms of a natural cup product, and if the results outlined above extend to singular metrics. We show this is indeed the case. at University of Iowa Libraries/Serials Acquisitions on June 1, 2015 http://imrn.oxfordjournals.org/ Downloaded from Hermitian-Holomorphic Deligne Cohomology 1017In this work, we first present an extension of the framework of Hermitianholomorphic Deligne cohomology to pairs (X, D), where X is a complete complex algebraic manifold and D is a normal crossing divisor. Then, by working on a regular projective curve over C, we use the cup product in this cohomology to obtain the pairing for two Hermitian line bundles with Hermitian metr...