  To further understand the nature of the optical counterparts associated with
Ultraluminous X-Ray sources (ULXs), we obtained far ultraviolet spectra of the
reported counterparts to the ULXs in NGC 1313, Holmberg II, NGC 5204, and M81.
The spectral resolution of the ACS prism spectra degrades from 300 at 1300 A to
40 at 1850 A, so longer wavelength features have lower S/N. The spectra of the
ULXs in NGC 1313, Ho II, and NGC 5204 are quite similar, showing the N V 1240 A
line at about the same equivalent width strength. The presence of this and
other emission lines confirms the presence of an accretion disk, probably
diluted by the light of an early B star companion. The spectra differ strongly
from high mass X-ray binaries dominated by O star winds (e.g., Cyg X-1) and are
most similar to intermediate mass X-ray binary systems in the Milky Way and
LMC. This indicates that the mass transfer is due to Roche Lobe overflow. The
spectrum of the ULX in M81 is quite weak but suggestive of a late-type
Wolf-Rayet star.
