  Binary systems consisting of a white dwarf and a main-sequence companion with
orbital periods up to $\approx 100$ d are often thought to be formed through
common envelope evolution which is still poorly understood. To provide new
observational constraints on the physical processes involved in the formation
of these objects, we are conducting a large-scale survey of close binaries
consisting of a white dwarf and an A to K-type companion. Here we present three
systems with eccentric orbits and orbital periods between $\approx10-42$ d
discovered by our survey. Based on HST spectroscopy and high angular resolution
images obtained with SPHERE-IRDIS, we find that two of these systems are most
likely triple systems while the remaining one could be either a binary or a
hierarchical triple but none of them is a post common envelope binary (PCEB).
The discovery of these systems shows that our survey is capable to detect
systems with orbital periods of the order of weeks, but all six PCEBs we have
previously discovered have periods below 2.5 d. We suggest that the fact that
all of the systems we identify with periods of the order of weeks are not PCEBs
indicates a transition between two different mechanisms responsible for the
formation of very close ($\lesssim 10$ d) and somewhat wider WD+AFGK binaries:
common envelope evolution and non-conservative stable mass transfer.
