Casimir temperature correction to the Bosonic string mass

Through the Casimir effect, we obtain a temperature correction to the ground state mass of the Bosonic string. Accordingly, we show that higher than the temperature $\approx 1/2\pi $, the Tachyon attain real mass, and also we find massive Graviton for nonzero temperatures. As another consequence, we demonstrate that for temperatures higher than the Hagedorn temperature, the string would find again a Tachyonic state.