On the Algebraic Structure of the Holomorphic Anomaly for N=2   Topological Strings
  The special geometry ($(t,{\bar t})$-equations) for twisted $N=2$ strings are derived as consistency conditions of a new contact term algebra. The dilaton appears in the contact terms of topological and antitopological operators. The holomorphic anomaly, which can be interpreted as measuring the background dependence, is obtained from the contact algebra relations. 
C. Gomez and E. Lopez