Key points are not available for this paper at this time.
Recently a technique for extending general relativity called algebraic extension was shown to yield only five classes of gravitational theories (general relativity plus four extensions). The particle spectra of these theories are analysed and it is shown that only one of these extensions is ghost free. Two inequivalent theories are shown to result from this extension at the linearised level. One of these is the linearised version of Moffat's theory of gravitation (1979) the other is a new theory which possesses an additional gauge invariance which has been associated with a closed string.
Kelly et al. (1986) studied this question.