  We present a measurement of lopsidedness for the stellar disks of 60 field
spiral galaxies in terms of the azimuthal m=1 Fourier amplitude, A_1, of the
stellar light. We conjecture that this lopsidedness is caused by tidal
interactions and calculate an UPPER limit on the accretion rate of small
galaxies. We exploit the correlation between lopsidedness and photometric
measures of recent star formation (Zaritsky 1995) to obtain two independent
estimates of the lifetime of these m=1 distortions. First, we show that
lopsided galaxies have an excess of blue luminosity relative to that of
symmetric galaxies with the same H I linewidth, which we attribute to a recent
star formation episode that was triggered by an interaction between the galaxy
and a companion. We use stellar populations models (Bruzual and Charlot 1993)
to estimate the time since that interaction. Second, we use the N-body
simulation of an infalling satellite by Walker, Mihos, and Hernquist (1996) to
estimate how fast tidally induced m=1 distortions are erased through phase
mixing. Both approaches indicate that the observations are consistent with a
hypothesized tidal interaction that occurred about 1 Gyr ago for galaxies that
are lopsided at the 20% level. By combining this lifetime estimate for
lopsidedness, the observed frequency of such distortions, and a correction to
the survey volume that depends on the increase in luminosity during an
interaction, we derive an upper limit on the current companion accretion rate
of field spiral galaxies (for companion masses of about 10% parent galaxy mass)
that lies in the range 0.07 - 0.25 per Gyr. The principal uncertainty in this
limit arises from ambiguities in the interpretation of the correlation between
lopsidedness and M_B.
