The dawn of a new era for dustless HdC stars with GAIA eDR3
  Decades after their discovery, only four hydrogen-deficient carbon (HdC)
stars were known to have no circumstellar dust shell. This is in complete
contrast to the $\sim$130 known Galactic HdC stars that are notorious for being
heavy dust producers, i.e. the R Coronae Borealis (RCB) stars. Together they
form a rare class of supergiant stars that are thought to originate from the
merger of CO/He white dwarf binary systems, otherwise known as the
double-degenerate scenario. We searched for new dustless HdC (dLHdC) stars. We
used primarily the 2MASS and GAIA eDR3 catalogues to select candidates that
were followed-up spectroscopically. We discovered 27 new dLHdC stars, one new
RCB star and two new EHe stars. Surprisingly, 20 of the new dLHdC stars share a
characteristic of the known dLHdC star HD 148839, having lower atmospheric
hydrogen deficiencies. The uncovered population of dLHdC stars exhibit a
Bulge-like distribution, like the RCB stars, but show multiple differences from
those that indicate they are a different population of HdC stars following its
own evolutionary sequence with a fainter luminosity and also a narrow range of
effective temperature. We found indication of a current low dust production
activity for four of the new dLHdC stars which could be typical RCB stars
passing through a transition time. We have evidence for the first time of a
large range of absolute magnitudes in the overall population of HdC stars,
spanning over 3 mag. In the favoured formation framework, this is explained by
a large range in the initial total WD binary mass which leads to a series of
evolutionary sequences with distinct maximum brightness and initial
temperature. The cold Galactic RCB stars are also noticeably fainter than the
Magellanic ones, possibly due to a difference in metallicity resulting in
different WD mass ratio. In our Galaxy, there could be as many dLHdC stars as
RCB stars.
