Cataclysmic Variables and a Candidate Helium White Dwarf in the Globular Cluster NGC 6397

We have used HST/FOS to study faint UV stars in the core of the nearbyglobular cluster NGC 6397. We confirm the presence of a 4th cataclysmicvariable (CV) in NGC 6397 (CV 4), and we use the photometry of Cool et al.(1998) to present evidence that CVs 1--4 all have faint disks and probably lowaccretion rates. By combining these results with new UV spectra of CV 1 and thepublished spectra of Grindlay et al. (1995) we present new evidence that CVs1--3 may be DQ Her systems, and we show that CV 4 may either be a dwarf nova oranother magnetic system. Another possibility is that the CVs could be old novaein hibernation between nova eruptions. We also present the first spectrum of amember of a new class of UV bright stars in NGC 6397. These faint, hot stars donot vary, unlike the CVs, and are thus denoted as ``non-flickerers'' (NFs).Like the CVs, their spatial concentration is strongly concentrated toward thecluster center. Using stellar atmosphere models we have determined log g =6.25, and T_eff = 17,500 K for this NF. Using these line parameters and theluminosity of the NF we show that the NF spectrum is consistent with a heliumWD having a mass of ~0.25 solar masses and an age between 0.1 and 0.5 Gyr(depending on the models used). The NF spectrum appears to be significantlyDoppler shifted from the expected wavelength, suggesting the presence of adark, massive companion, probably a carbon-oxygen WD.Comment: 29 pages, 8 figures; to appear in the Astrophysical Journa