A general symmetry of the bilinear BKP hierarchy is studied in terms of tau
functions. We use this symmetry to define reductions of the BKP hierarchy,
among which new integrable systems can be found. The reductions are connec
ted to constraints on the Lax operator as well as on the bilinear formulati
on. A class of solutions for the reduced equations is derived. (C) 1999 Ame
rican Institute of Physics. [S0022-2488(99)02603-1].