  A new analysis of Keck/HIRES observations of the broad absorption line QSO
APM 0827+5255 indicates that a number of intervening C IV absorbers give rise
to absorption lines for which the observed optical depths for 1548, 1550 A
doublet components are not in the expected 2:1 ratio. To compensate for the
effect, a local adjustment of the zero-level is required. We model this effect
as coverage of one line of sight to this gravitationally lensed QSO and perform
a set of simulations to select a sample of lines for which our model provides
an explanation for the effect. We use lines in this sample to obtain estimates
for minimum C IV absorber sizes from total coverage and the separations of the
lines of sight for a range of lens redshifts, z_l, and two cosmologies. We also
obtain best estimates for overall sizes from a statistical `hit and miss'
approach. For z_l=0.7 our results set a lower limit to sizes of C IV absorbers
of ~0.3 h_72^{-1} kpc (~0.5 h_72^{-1} kpc) for Omega_M=1, Omega_Lambda=0
(Omega_M=0.3, Omega_Lambda=0.7), in agreement with other results from similar
work but are limited by sample size and the uncertainty in z_l. Our method can
be used to detect lensed QSOs and to probe absorber sizes when separate spectra
cannot be obtained for each line of sight.
