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We present the multiloop partition function of open bosonic string theory inthe presence of a constant gauge field strength, and discuss its low-energylimit. The result is written in terms of twisted determinants and differentialson higher-genus Riemann surfaces, for which we provide an explicitrepresentation in the Schottky parametrization. In the field theory limit, werecover from the string formula the two-loop Euler-Heisenberg effective actionfor adjoint scalars minimally coupled to the background gauge field.Comment: 32 pages, 3 eps figures, plain LaTeX. References added, minor changes  to the text. Published version, affiliation correcte

TWO-LOOP EULER–HEISENBERG EFFECTIVE ACTIONS FROM CHARGED OPEN STRINGS