Z Andromedae is the class prototype of the symbiotic stars, binary stars consisting of a red giant donor star and a hot accreting star, either a white dwarf or neutron star. The most prominent features of the Z Andromedae light curve shown here are the outbursts that occur roughly every 600 days. In the case of Z Andromedae, the optical light of these outbursts arises from two processes: an outburst of an accretion disk around the white dwarf, and from thermonuclear burning on the surface of the white dwarf itself. The latter is a similar process as what happens in the classical novae, but at a much slower rate.
The outbursts of Z Andromedae do not occur with the same frequency as the orbital period -- recent outbursts have occurred between 600 and 700 days apart, but the orbital period is slightly longer, about 750 days. Leibowitz and Formiggini (2008) suggest that the outburst period is a beat period between this orbital period and a longer cycle, perhaps similar to the 11-year solar-activity cycle in the donor star.