[en] Undulator magnets are used in connection with particle (typically electron) storage rings to enhance the spectrum of synchrotron radiation and are a basic component of free electron lasers. Two configurations are common: helical undulators, for which the magnetic field on the axis is constant in modulus and rotates around the axis as we proceed along it; and transverse undulators, that have a median plane of symmetry. The orientation of the field for a transverse geometry is always the same on the median plane, but the modulus of the field is spatially modulated. For the electron storage rings that constitute the National Synchrotron Light Source at Brookhaven, undulators have been proposed to produce a line spectrum of spontaneous radiation to be used in the experiments. One of these undulators will be used for the free electron laser experiment on the VUV-ring. The basic design of this undulator and its expected performance are discussed