RAPID ULTRAVIOLET VARIABILITY IN THE BL LACERTAE OBJECT PKS-2155-304

This Letter reports the first clear evidence of a rapid change in the ultraviolet flux of a BL Lac object. Sophisticated analysis of 57 closely spaced IUE spectra yielded clear evidence of a approximately 12% drop in the ultraviolet flux of PKS 2155-304 over a 2 day period in 1989 October and a strong indication of a similar drop in a 5 hr period some three weeks later. These are near the fastest variations which could have been detected with these data, but they are still much slower (or weaker) variations than have been observed in the X-rays. However, they are orders of magnitude faster than Seyfert 1 ultraviolet variations. This suggests that a "standard" Seyfert 1 accretion disk may not be responsible for the ultraviolet emission from PKS 2155-304.