  In spite of their importance for modern astronomy, we do not fully understand
how close binary stars containing at least one white dwarf form from main
sequence binary stars. The discovery of EL CVn binaries, close pre-white dwarfs
with A/F main sequence star companions, offers now the unique possibility to
test models of close compact binary star formation. Binary evolution theories
predict that these EL CVn stars descend from very close main sequence binaries
with orbital periods shorter than 3 days. If this is correct, nearly all EL CVn
stars should be inner binaries of hierarchical triples because more than 95 per
cent of very close main sequence binaries (the alleged progenitor systems) are
found to be hierarchical triples. We here present SPHERE/IRDIS observations of
five EL CVn binaries, finding in all of them tertiary objects, as predicted. We
conclude that EL CVn systems are inner binaries of hierarchical triples and
indeed descend from very close main sequence binaries that experience stable
mass transfer.
