In his book Mathematical Cosmology and Extragalactic Astronomy (Academic Press 1976) (pages 72-75, 88-91), Segal says:
"... The suitably scaled 15 linearly independent generators Lij of symmetries of unispace [Segal's term for the Conformal RP1 x S3 SpaceTime used in the D4-D5-E6-E7 model - from here on on this page I will call it Conformal SpaceTime] ... differ from the 11 generators of the group of global conformal transformations in Minkowski space by terms of the order 1 / R^2 [as R becomes infinite, where R is the radius of curvature of Conformal SpaceTime] ...
... The angular momenta Lij ... [ i,j = 1,2; 2,3; 3,1 ] ... have ... the same expression both in Minkowski space and in [Conformal SpaceTime] ... The same is true of the boosts ... -iL0,j ... ( j = 1,2,3 ) ... and the infinitesimal scale transformation [ L-1,4 ] ...
... The scale generator - L-1,4 ... determines a ... scalar field. This ... is most naturally interpreted from a gravitational standpoint ...
... two ordered sets, each containing four of the Lij, converge on the same ... fields in Minkowski space ... in particular, R^(-1) L-1,j and R^(-1) Lj,4 [ for j = 0,1,2,3 ] both ... agree ... [as R becomes infinite] with the [corresponding Minkowski] conventional energy-momentum component. ... The differences L-1,j - Lj,4 thus are ... representable by a ... vector field, which physically would appear most naturally as potentially related to gravitational phenomena ... ".
...let the Scalar Field determined by the scale generator - L-1,4 correspond to the Higgs Scalar Field. Then, let, for j = 0,1,2,3, the four generators L-1,j - Lj,4 represent a vector field, what I call the GraviPhoton field. GraviPhotons look like: Virtual Covariant Conventional U(1) Photons, in that they have 4 Components, including Longitudina/Scalar Components; and Vector Gravitons that can interact with the Imaginary Part of Complex Spacetime. The relevant Complex Structure can be seen in such physical concepts as Momentum Space, Position-Momentum Complementarity, Type IV(2) Domains, Hyperspace, Black Holes, and Wavelets.