Thermofield Dynamics of the Closed Bosonic String - Physical Aspects of   the Thermal Duality -
  The thermofield dynamics of the D=26 closed bosonic thermal string theory is described in proper reference to the thermal duality symmetry as well as the thermal stability of modular invariance in association with the global phase structure of the bosonic thermal string ensemble. 
H. Fujisaki and K. Nakagawa