DILUTE D-INSTANTONS AT FINITE-TEMPERATURE

We discuss Dirichlet instanton effects on type-IIB string thermodynami cs. 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 interac tions. Thermal singularities in the single-instanton sector are due to tachyonic winding modes of Dirichlet open strings, Purely bosonic D-i nstantons 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 temperatur e, and the induced mass and coupling of the axion field are insufficie nt to change the first-order character of the phase transition in the mean field approximation. (C) 1997 Elsevier Science B.V.