The Quasi-Cepheid Nature of Rho-Cassiopeiae

Rho Cassiopeiae (HR 9045, HD 224014) is a bright yellow supergiant (F8pla). If it is a member of the association CAS 0B5, it has an Mv of about -9.5, and is therefore one of the most luminous yellow supergiants known (Humphreys 1978).
The brightness of ρ Cas varies in two ways: (i) it undergoes semiregular cycles with amplitudes of about 0.2 and time scales of about a year, perhaps due to pulsation, and (ii) in the 1940’s, it faded by more than a magnitude for 660 days (Gaposchkin 1949); as a result, it is often classified as “R CrB?”. Although various writers have pointed to a dissimilarities between ρ Cas and the R CrB stars, it should be noted that R CrB itself undergoes semi-regular cycles as well as fadings. Thus the scientific interest of ρ Cas lies in the nature of its variations, their possible relationship to each other and to the mass loss and extreme luminosity of the star.