Partial linear spaces built on hexagons

We define four families of geometries with as point graph the graph — or its complement — of all elliptic 4 hyperplanes of a given parabolic quadric in any finite 6-dimensional projective space, where adjacency is 5 given by intersecting in a tangent 4-space. One of the classes consists of semi-partial geometries constructed 6 in J.A. Thas [SPG-reguli and semipartial geometries, Adv. Geom. 1 (2001) 229–244], for which our 7 approach yields a new construction, more directly linked to the split Cayley hexagon. Our main results 8 determine the complete automorphism groups of all these geometries. 9 c © 2005 Published by Elsevier Ltd 10