  The common envelope (CE) interaction describes the swallowing of a nearby
companion by a growing, evolving star. CEs that take place during the
asymptotic giant branch phase of the primary and may lead to the formation of a
planetary nebula (PN) with a post-CE close binary in the middle. We have used
published observations of masses and kinematics of jets in four post-CE PN to
infer physical characteristics of the CE interaction. In three of the four
systems studied, Abell 63, ETHOS 1 and the Necklace PN, the kinematics indicate
that the jets were launched a few thousand years before the CE and we favour a
scenario where this happened before Roche lobe overflow, although better models
of wind accretion and wind Roche lobe overflow are needed. The magnetic fields
inferred to launch pre-CE jets are of the order of a few Gauss. In the fourth
case, NGC 6778, the kinematics indicate that the jets were launched about 3000
years after the CE interaction. Magnetic fields of the order of a few hundreds
to a few thousands Gauss are inferred in this case, approximately in line with
predictions of post-CE magnetic fields. However, we remark that in the case of
this system, it is impossible to find a reasonable scenario for the formation
of the two jet pairs observed: the small orbital separation would preclude the
formation of even one accretion disk able to supply the necessary accretion
rate to cause the observed jets.
