Dilute D-Instantons at Finite Temperature

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 which ruin the weak coupling expansion in the microcanonical ensemble. Type IIB D-instantons however 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.