HL CANIS-MAJORIS IN PREOUTBURST AND SS CYGNI - THE INTEROUTBURST DISKINSTABILITY

SS Cygni and HL Canis Majoris were observed by IUE for three consecuti ve nights in November of 1992. During the first two nights, simultaneo us photometric ground-based observations of SS Cyg were made at the Ba ll State University Observatory. Observations of SS Cyg and HL CMa wer e also obtained simultaneously with the 90-inch telescope at the Stewa rd Observatory on the last two nights of the IUE run. These spectrosco pic observations covered the wavelength range from 4 1 00 to 5000 angs trom, while the spectra taken with the short wavelength camera on IUE resulted in wavelength coverage from 1150 angstrom to 1980 A. SS Cyg i s a U Geminorum-type dwarf nova with an orbital period of 6.6 hr. Good simultaneous UV and optical orbital coverage was obtained for this sy stem. HL CMa is a Z Camelopardalis-type dwarf nova with a mean outburs t interval of 15 days. The AAVSO reports that this system was in outbu rst 4 days after the observing run. Therefore, HL CMa may have been in a preoutburst state during these observations. Optical spectra of HL CMa indicate a warm front passed through the outer disk four days befo re outburst, but no changes were seen in the UV spectra. Signs of a pr eoutburst state were observed to develop in SS Cyg, but no outburst oc curred for another 30 days.