  We propose that supermassive stars may form in quasar accretion disks, and we
discuss possible observational consequences. The structure and stability of
very massive stars are reviewed. Because of high accretion rates, quasar disks
are massive and the fringes of their optically luminous parts are prone to
fragmentation. Starting from a few hundred solar masses, a dominant fragment
will grow to the isolation mass, which is a significant fraction of the disk
mass, more quickly than the fragment contracts onto the stellar main sequence.
A gap will form in the disk and the star will migrate inward on the accretion
timescale, which is comparable to the star's main sequence lifetime. By
interrupting the gas supply to the inner disk, the gap may temporarily dim and
redden the quasar. The final stages of stellar migration will be a strong
source of low-frequency gravitational waves.
