TESS uncloaks the secondaries in hydrogen-deficient binaries

TL;DR

TESS observations of hydrogen-deficient binaries reveal short-period flux variations likely linked to secondary star rotation or oscillations, providing insights into the secondary's properties and the primary's stripping process.

Contribution

This study directly detects the secondary stars in four hydrogen-deficient binaries using TESS data, offering new understanding of their rotation and surface features.

Findings

Detected flux variations with 0.5-0.9 day periods in all four systems.

Supported the secondary's rotation period hypothesis.

Provided direct observational evidence of the secondary stars.

Abstract

$\upsilon$ Sgr is the prototype of four known hydrogen-deficient binary (HdB) systems. These are characterised by a hydrogen-deficient A-type primary, variable hydrogen emission lines, and a normally unseen secondary presumed to be an upper main-sequence star. Orbital periods range from tens of days to 360 d. TESS observations of all four HdBs show a flux variation with well-defined period in the range 0.5 -- 0.9 d, too short to be associated with the supergiant primary, and more likely to be the rotation period of the secondary and associated with a chemical surface asymmetry or a low-order non-radial oscillation. The observed rotation period supports a recent analysis of the $\upsilon$ Sgr secondary. The observations give a direct glimpse of the secondary in all four systems, and should help to explain how the primary has been stripped to become a low-mass hydrogen remnant.