Thermal ensemble of string gas in a magnetic field

We study the thermal ensemble of a gas of free strings in presence of a magnetic field. We find that the thermodynamical partition function diverges when the magnetic field exceeds some critical value $B_{\rm cr}$, which depends on the temperature. We argue that there is a first-order phase transition with a large latent heat. At the critical value an infinite number of states -all states in a Regge trajectory- seem to become massless, which may be an indication of recuperation of spontaneously broken symmetries.