COVARIANCE PROPERTIES OF HIGHER GENUS TWIST CORRELATORS UNDER R[--]1/R DUALITY IN ORBIFOLD CONFORMAL FIELD-THEORY

We study R↔ 1 R duality in simple orbifold models at arbitrary orders in string perturbation theory. It is shown that duality involves certain linear relations among twist correlators at dual radii. We derive general expressions for the coefficients of these relations and show that they can be expressed in terms of functions of the orbifold cosets obeying an algebra bearing a striking resemblance to Verlinde's fusion rule algebra. Finally, we study the behavior of the coefficients under factorization, in particular their dependence on the genus. A few examples are worked out. © 1990.