Disk instabilities in Hubble-Sandage variables

The brightest individual objects in extragalactic nebulae are the Hubble-Sandage variables. They were first investigated by Hubble and Sandage (1953). Observations by Tamman and Sandage (1968) and Rosino and Bianchini (1973) followed. The main characteristics are high luminosity (L/L(Sun) approximately 105), blue color indices, F type spectra and irregular variability. Bath (1979) has suggested that the Hubble-Sandage variables contain an accreting main-sequence star with a Roche Lobe filling companion in a wide binary system. Based on this model the authors derive theoretical color indices for disks and determine the mass in the disk for different mass accretion rates. Further they discuss an instability of the disk which could explain the change in the color index observed for Var A in M33. (Auth.)