Seven-superform gauge fields in N=1, D=10 supergravity and duality

We present a formulation of N = 1, D = 10 supergravity-super-Maxwell theory in superspace in which the graviphoton can be described by a 2-form B2 or a 6-form B6, the photon by a 1-form A1 or a 7-form A7 and the dilaton by a scalar φ or an 8-form φ8, the supercurvatures of these fields being related by duality. Duality interchanges Bianchi identities and equations of motion for each of the three couples of fields. This construction envisages the reformulation of D = 10 supergravity, involving 7-forms as gauge fields, conjectured by Schwarz and Sen, which, upon toroidal compactification to four dimensions, gives the manifestly SL(2, R)s invariant form of the heterotic string effective action.