Are white dwarf magnetic fields in close binaries generated during
  common-envelope evolution?
  Understanding the origin of the magnetic fields in white dwarfs (WDs) has
been a puzzle for decades. A scenario that has gained considerable attention in
the past years assumes that such magnetic fields are generated through a dynamo
process during common-envelope evolution. We performed binary population models
using an up-to-date version of the BSE code to confront the predictions of this
model with observational results. We found that this hypothesis can only
explain the observed distribution of WD magnetic fields in polars and
pre-polars and the low-temperature WDs in pre-polars if it is re-scaled to fit
the observational data. Furthermore, in its present version, the model fails to
explain the absence of young close detached WD+M-dwarf binaries harbouring hot
magnetic WDs and predicts that the overwhelming majority of WDs in close
binaries should be strongly magnetic, which is also in serious conflict with
the observations. We conclude that either the common-envelope dynamo scenario
needs to be substantially revised or a different mechanism is responsible for
the generation of strong WD magnetic fields in close binaries.
