The evolutionary status of the hot R CrB stars has long remained a matter of conjecture,
primarily because of the relative dearth of relevant observational material. Previously the group had been thought to occupy a position intermediate in status between the EHe and R CrB classes as they have (at least at first glance) observational properties in common with both types of object.
The photosphere of DY Cen has been quantitatively confirmed to be hydrogen-deficient
and photometric variations suggest the star undergoes short period pulsations. Using a
period-temperature relation applicable to hydrogen-deficient stars it would appear that this object has similar physical properties to both the EHe and R CrB stars.
Narrow band imaging of V348 Sgr has shown that the associated nebula exhibits a bipolar structure and therefore must be closely related to planetary nebulae rather than H II regions. Spectroscopic observations have proved that the star in its present evolutionary state is incapable of ionising the nebula. Several scenarios for this behaviour are briefly discussed. The large helium enrichment found in the nebula indicates that processed material must have been ejected during the last major episode of mass loss. Evidence is presented that suggests a strong hydrogen abundance gradient exists within the nebula.
A novel technique has been developed for determining reddening distances. Its main advantage over other similar methods is that both early and late-type stars may be used to establish the reddening-distance relationship. With more development this technique may prove to be an important tool in distance determinations for objects such as planetary nebulae etc. This technique was used to derive a distance of (4. 7±1.0) kpc for V348 Sgr. Using the core-mass relation for hydrogen-deficient stars implies that both V348 Sgr and MV Sgr are lower mass and luminosity objects than EHe and R CrB stars.
The evidence presented in this thesis indicates that the hot R CrB group is not a homogeneous one. DY Cen is much more luminous and massive than the other members. The mass and luminosity of V348 Sgr and MV Sgr are consistent with the scenario that both have recently suffered a thermal pulse (causing re-ignition of a helium burning shell) and are currently looping back to the R CrB domain of the H R diagram
