  The precise origin of short gamma ray bursts (SGRBs) remains an important
open question in relativistic astrophysics. Increasingly, observational
evidence suggests the merger of a binary compact object system as the source
for most SGRBs, but it is currently unclear how to distinguish observationally
between a binary neutron star progenitor and a black hole-neutron star
progenitor. We suggest the quasi-periodic signal of jet precession as an
observational signature of SGRBs originating in mixed binary systems, and
quantify both the fraction of mixed binaries capable of producing SGRBs, and
the distributions of precession amplitudes and periods. The difficulty inherent
in disrupting a neutron star outside the horizon of a stellar-mass black hole
biases the jet precession signal towards low amplitude and high frequency.
Precession periods of ~ 0.01-0.1 s and disk-BH spin misalignments ~10 degrees
are generally expected, although sufficiently high viscosity may prevent the
accumulation of multiple precession periods during the SGRB. The precessing jet
will naturally cover a larger solid angle in the sky than would standard SGRB
jets, enhancing observability for both prompt emission and optical afterglows.
