  The R Coronae Borealis (RCB) stars are cool supergiants that display
irregular and deep dips in their light curves, caused by dust formation. There
are four known hot RCB stars (DY Cen, MV Sgr, V348 Sgr, and HV 2671), with
surface temperatures of 15,000--25,000 K, and prior work has suggested that
three of these have secular fading in brightness. I have tested this result by
measuring century-long light curves in the Johnson B-band with modern
comparison star magnitudes, and I have extended this by measuring many
magnitudes over a wide time range as well as for the fourth hot RCB star. In
all four cases, the B-band magnitude of the maximum light is now fast fading.
The fading rates (in units of magnitudes per century) are 2.5 for DY Cen after
1960, 1.3 for MV Sgr, 1.3 for V348 Sgr, and 0.7 for HV 2671. This secular
fading is caused by the expected evolution of the star across the top of the HR
diagram at constant luminosity, as the temperature rises and the bolometric
correction changes. For DY Cen, the brightness at maximum light is rising from
1906 to 1932, and this is caused by the temperature increase from near 5,800 to
7,500 K. Before 1934 DY Cen had frequent dust dips, while after 1934 there are
zero dust dips, so there is some apparent connection between the rising
temperature and the formation of the dust. Thus, we have watched DY Cen evolve
from an ordinary RCB star up to a hot RCB star and now appearing as an extreme
helium star, all in under one century.
