  We present the results of an expanded, long-term radial velocity search (25
yrs) for evidence of binarity in a sample of seven bright proto-planetary
nebulae (PPNe). The goal is to investigate the widely-held view that the
bipolar or point-symmetric shapes of planetary nebulae (PNe) and PPNe are due
to binary interactions. Observations from three observatories were combined
from 2007-2015 to search for variations on the order of a few years and then
combined with earlier observations from 1991-1995 to search for variations on
the order of decades. All seven show velocity variations due to periodic
pulsation in the range of 35-135 days. However, in only one PPN, IRAS
22272+5435, did we find even marginal evidence found for multi-year variations
that might be due to a binary companion. This object shows
marginally-significant evidence of a two-year period of low semi-amplitude
which could be due to a low-mass companion, and it also displays some evidence
of a much longer period of >30 years. The absence of evidence in the other six
objects for long-period radial velocity variations due to a binary companion
sets significant constraints on the properties of any undetected binary
companions: they must be of low mass, <0.2 M(sun), or long period, >30 years.
Thus the present observations do not provide direct support for the binary
hypothesis to explain the shapes of PNe and PPNe and severely constrains the
properties of any such undetected companions.
