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In a previous paper, we have defined arithmetic extension groups in the context of Arakelov geometry. In the present one, we introduce an arithmetic analogue of the Atiyah extension, that defines an element -- the arithmetic Atiyah class -- in a suitable arithmetic extension group. If $\overline{E}$ is a hermitian vector bundle on an arithmetic scheme $X$, its arithmetic Atiyah class is an obstruction to the algebraicity of the unitary connection on the vector bundle $E_\C$ over the complex manifold $X(\C)$ that is compatible with its holomorphic structure. We develop basic properties of the arithmetic Atiyah class and study its vanishing in the case of hermitian line bundles. This may be translated into a concrete problem of diophantine geometry, concerning rational points of the universal vector extension of the Picard variety of $X$. We investigate this problem, which was already considered and solved in some cases by Bertrand, by using a classical transcendence result of Schneider-Lang, and we derive a finiteness result. We also consider a geometric analog of our arithmetic situation, namely a smooth, projective variety $X$ which is fibered on a curve $C$ defined over some field $k$ of characteristic zero. To any line bundle $L$ over $X$ is attached its relative Atiyah class ${\rm at}_{X/C}L$. We describe precisely when this class vanishes. In particular, when the fixed part of the relative Picard variety of $X$ over $C$ is trivial, this holds only when the restriction of $L$ to the generic fiber $X_K$ of $X$ over $C$ is a torsion line bundle. |