quantum vacuum effects for a massive bosonic string in background b-field

We study the casimir effect for a bosonic string extended between dbranes, and living in a flat space with an antisymmetric background bfield. we find the casimir energy as a function of the bfield, and the massparameter of the string, and accordingly we obtain a bdependence correction term to the groundstate mass of the string. we show that for sufficiently large bfield, the ground state of the string contains real (i.e. nontachyonic) particles