  $\upsilon$ 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.
