  Post-common envelope binary systems evolve when matter is transferred from
the primary star at a rate that cannot be accommodated by its secondary
companion. A common envelope forms which is subsequently ejected resulting in a
system with a binary period frequently between 2 and 3 hours. Where
circumbinary companions are predicted, it remains unclear whether they form
before or after the common envelope ejection. From observations of eclipse time
variations (ETVs), exoplanet databases e.g. NASA Exoplanet Archive, list
typically a dozen systems with confirmed circumbinary planets. Here we examine
seven of these systems, discuss other possible causes and consider whether, for
these dynamic systems, the ETV methodology is a reliable indicator of planetary
companions. The systems selected were those where we could determine precise
eclipse timings, free from significant extraneous effects such as pulsations,
and present 163 new times of minima permitting us to test existing models. Over
thirty circumbinary models have been proposed for these seven systems and note
all, other than the latest model for NY Vir which remains to be fully tested,
fail within a year to accurately predict eclipse times. In examining
alternative mechanisms we find that magnetic effects could contribute
significantly in two of the seven systems studied. We conclude that the
structure of these dynamic systems, with the extreme temperature differences
and small binary separations, are not fully understood and that many factors
may contribute to the observed ETVs.
