Most EL CVn systems are inner binaries of hierarchical triples

TL;DR

This study confirms that most EL CVn binary systems are actually inner binaries within hierarchical triple systems, supporting models where close main sequence binaries evolve into such configurations through stable mass transfer.

Contribution

The paper provides observational evidence that all examined EL CVn systems are part of hierarchical triples, validating theories of their formation from very close main sequence binaries.

Findings

All five observed EL CVn systems have tertiary companions.

EL CVn binaries originate from very close main sequence binaries.

Supports models of stable mass transfer in hierarchical triples.

Abstract

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.