  The Thermo Field Dynamics formalism is presented. In particular, it is
applied to the two-dimensional field theory that describes a open bosonic
string. The value of entropy operator is computed in various Dirichlet and
Neumann boundary states. The Thermal Bogoliubov transformations, the thermal
vacuum and the thermal Fock space are computed explicitely for the case of
SU(1,1) thermal algebra.
