  T-duality is one of the essential elements of string theory. Recently, Hull
has developed a formalism where the dimension of the target space is doubled so
as to make T-duality manifest. This is then supplemented with a constraint
equation that allows the connection to the usual string sigma model. This paper
analyses the partition function of the doubled formalism by interpreting the
constraint equation as that of a chiral scalar and then using holomorphic
factorisation techniques to determine the partition function. We find there is
quantum equivalence to the ordinary string once the topological interaction
term is included.
