  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.
