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Mixed invariants are used to classify the Riemann spinor in the case ofEinstein-Maxwell fields and perfect fluids. In the Einstein-Maxwell case thesemixed invariants provide information as to the relative orientation of thegravitational and electromagnetic principal null directions. Consideration ofthe perfect fluid case leads to some results about the behaviour of theBel-Robinson tensor regarded as a quartic form on unit timelike vectors.Comment: 31 pages, AMS-LaTe

Geometric interpretation of the mixed invariants of the Riemann spinor