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We analyze ultraviolet (∼ 1500Å) images of the old open clusters M67, NGC 188, and NGC 6791 obtained with Ultraviolet Imaging Telescope (UIT) during the second flight of the Astro observatory in March 1995. Twenty stars are detected on the UIT image of M67, including 11 blue stragglers, seven white dwarf candidates, and the yellow giant -white dwarf binary S1040. The ultraviolet photometry of the blue stragglers F90 (S975) and F131 (S1082) suggests that these stars have hot subluminous companions. We present a semi-empirical integrated ultraviolet spectrum of M67, and show that the blue stragglers dominate the integrated spectrum at wavelengths shorter than 2600Å. The number of white dwarfs in M67 is roughly consistent with the number expected from white dwarf cooling models. Eight candidate sdB/sdO stars are detected in NGC 6791, and two are detected in NGC 188. The luminosity range 1.10 < log L/L ⊙ < 1.27, derived from the ultraviolet photometry of the six sdB candidates, is consistent with theoretical models of metal-rich hot horizontal branch (HB) stars. The fraction of hot HB stars in both NGC 6791 and NGC 188 is about 30%, implying that the integrated spectra of both clusters should show a UV turnup at least as strong as that observed in any elliptical galaxy.

The globular cluster u Centauri contains the largest known population of very hot horizontal-branch (HB) stars. We have used the Hubble Space T elescope to obtain a far-UV/optical color-magnitude diagram of three Ðelds in u Cen. We Ðnd that over 30% of the HB objects are "" extreme ÏÏ HB or hot post-HB stars. The hot HB stars are not concentrated toward the cluster center, which argues against a dynamical origin for them. A wide gap in the color distribution of the hot HB stars appears to correspond to gaps found earlier in several other clusters. This suggests a common mechanism, probably related to giant branch mass loss. The diagram contains a signiÐcant population of hot sub-HB stars, which we interpret as the "" blue-hook ÏÏ objects previously predicted by DÏCruz et al. These are produced by late He Ñashes in stars which have undergone unusually large giant branch mass loss. The cluster u Cen has a well-known spread of metal abundance, and our observations are consistent with a giant branch mass-loss efficiency which increases with metallicity.

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