What is the highest number of symmetries (Killing vectors) that a (4-dimensional) spacetime can have without being maximally symmetric? From what I can see, it seems to be 7 (which includes the Einstein universe and some pp-wave spacetimes), but the theorems used (in chapter 11 and 12 of Stephani's "Exact Solutions of the Einstein's Field Equations") only apply to spacetimes stemming from a variety of stress energy tensors (Vacuum, lambda vacuum, EM fields, perfect fluids, pure radiation).

Can that result be generalized to all spacetimes, regardless of their sources?

This post imported from StackExchange Physics at 2015-08-17 08:04 (UTC), posted by SE-user Slereah