Dilute D-Instantons at Finite Temperature

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We discuss Dirichlet instanton effects on type-IIB string Thermodynamics. We review some general properties of dilute D-instanton gases and use the low-energy supergravity solutions to define the normalization of the instanton measure, as well as the effects of long-range interactions. Thermal singularities in the single-instanton sector are due to tachyonic winding modes of Dirichlet open strings. Purely bosonic D-instantons induce in this way hard infrared singularities that ruin the weak-coupling expansion in the microcanonical ensemble. However, type-IIB D-instantons, give smooth contributions at the Hagedorn temperature, and the induced mass and coupling of the axion field are insufficient to change the first-order character of the phase transition in the mean field approximation.