  We solve the Killing spinor equations of supersymmetric IIB backgrounds which
admit one supersymmetry and the Killing spinor has stability subgroup G_2 in
Spin(9,1) x U(1). We find that such backgrounds admit a time-like Killing
vector field and the geometric structure of the spacetime reduces from
Spin(9,1) x U(1) to G_2. We determine the type of G_2 structure that the
spacetime admits by computing the covariant derivatives of the spacetime forms
associated with the Killing spinor bilinears.
  We also solve the Killing spinor equations of backgrounds with two
supersymmetries and Spin(7)\ltimes R^8-invariant spinors, and four
supersymmetries with SU(4)\ltimes R^8- and with G_2-invariant spinors. We show
that the Killing spinor equations factorize in two sets, one involving the
geometry and the five-form flux, and the other the three-form flux and the
scalars. In the Spin(7)\ltimes R^8 and SU(4)\ltimes R^8 cases, the spacetime
admits a parallel null vector field and so the spacetime metric can be locally
described in terms of Penrose coordinates adapted to the associated rotation
free, null, geodesic congruence. The transverse space of the congruence is a
Spin(7) and a SU(4) holonomy manifold, respectively. In the G_2 case, all the
fluxes vanish and the spacetime is the product of a three-dimensional Minkowski
space with a holonomy G_2 manifold.
