Abstract:
The hydrogen-deficient star DY Cen has been reported as an R CrB-type variable, an extreme
helium star (with some hydrogen), and as a single-lined spectroscopic binary. It has been
associated with a dramatic change in visual brightness and colour corresponding to a change
in effective temperature ( Teff) of some 20 000 K in the last century. To characterize the binary
orbit and Teff changes more precisely, new high-resolution spectroscopy has been obtained
with SALT. The previous orbital period is not confirmed; previous measurements may have
been confused by the presence of pulsations. Including data from earlier epochs (1987, 2002,
and 2010), self-consistent spectral analyses from all four epochs demonstrate an increase
in Teff from 18 800 to 24 400 K between 1987 and 2015. Line profiles demonstrate that the
surface rotation has increased by a factor of 2 over the same interval. This is commensurate
with the change in Teff and an overall contraction. Rotation will exceed critical if contraction
continues. The 1987 spectrum shows evidence of a very high abundance of the s-process
element strontium. The very rapid evolution, non-negligible surface hydrogen and high surface
strontium point to a history involving a very late thermal pulse. Observations over the next 30
yr should look for a decreasing pulsation period, reactivation of R CrB-type activity as the star
seeks to shed angular momentum and increasing illumination by emission lines from nebular
material ejected in the past
