  We have used the tidal equations of Zahn to determine the maximum orbital
distance at which companions are brought into Roche lobe contact with their
giant primary, when the primary expands during the giant phases. This is a key
step when determining the rates of interaction between giants and their
companions. Our stellar structure calculations are presented as maximum radii
reached during the red and asymptotic giant branch (RGB and AGB, respectively)
stages of evolution for masses between 0.8 and 4.0 Mo (Z=0.001 - 0.04) and
compared with other models to gauge the uncertainty on radii deriving from
details of these calculations. We find overall tidal capture distances that are
typically 1-4 times the maximum radial extent of the giant star, where
companions are in the mass range from 1 Jupiter mass to a mass slightly smaller
than the mass of the primary. We find that only companions at initial orbital
separations between ~320 and ~630 Ro will be typically captured into a Roche
lobe-filling interaction or a common envelope on the AGB. Comparing these
limits with the period distribution for binaries that will make PN, we deduce
that in the standard scenario where all ~1-8 Mo stars make a PN, at most 2.5
per cent of all PN should have a post-common envelope central star binary, at
odds with the observational lower limit of 15-20 per cent. The observed
over-abundance of post-interaction central stars of PN cannot be easily
explained considering the uncertainties. We examine a range of explanations for
this discrepancy.
