A calculation of a partition function Z in a system of two coincident D1--\bar D1 pairs of type I superstring theory is presented. According to the well known conjecture, this partition function is identified with a tachyon potential in a case of constant tachyon fields. Properties of this potential are discussed. On the way, a peculiar features of  boundary fermions emerge. To solve the problems ensuing, a non-standard way of operator renormalization is required.

PACS numbers: 11.25.Sq