 For example, in the presence of Neveu–Schwarz H-flux or non-spin cycles some RR fluxes dictate the presence of D-branes.In the former case this is a consequence of the supergravity equation of motion which states that the product of a RR flux with the NS 3-form is a D-brane charge density.Thus the set of topologically distinct RR field strengths that can exist in brane-free configurations is only a subset of the cohomology with integral coefficients.