  We describe observational evidence for a new kind of interacting-binary-star
outburst that involves both an accretion instability and an increase in
thermonuclear shell burning on the surface of an accreting white dwarf. We
refer to this new type of eruption as a combination nova. In late 2000, the
prototypical symbiotic star Z Andromedae brightened by roughly two magnitudes
in the optical. We observed the outburst in the radio with the VLA and MERLIN,
in the optical both photometrically and spectroscopically, in the far
ultraviolet with FUSE, and in the X-rays with both Chandra and XMM. The
two-year-long event had three distinct stages. During the first stage, the
optical rise closely resembled an earlier, small outburst that was caused by an
accretion-disk instability. In the second stage, the hot component ejected an
optically thick shell of material. In the third stage, the shell cleared to
reveal a white dwarf whose luminosity remained on the order of 10^4 Lsun for
approximately one year. The eruption was thus too energetic to have been
powered by accretion alone. We propose that the initial burst of accretion was
large enough to trigger enhanced nuclear burning on the surface of the white
dwarf and the ejection of an optically thick shell of material. This outburst
therefore combined elements of both a dwarf nova and a classical nova. Our
results have implications for the long-standing problem of producing shell
flashes with short recurrence times on low-mass white dwarfs in symbiotic
stars.
