Discovery of a Magnetic White Dwarf in the Symbiotic Binary Z Andromedae

We report the first result from our survey of rapid variability in symbiotic binaries: the discovery of a persistent oscillation at P=1682.6+/-0.6 s in the optical emission from the prototype symbiotic Z Andromedae. The oscillation was detected on all eight occasions on which the source was observed over a time span of nearly 1 yr, making it the first such persistent periodic pulse found in a symbiotic binary. The amplitude was typically 2-5 mmag, and it was correlated with the optical brightness during a relatively small outburst of the system. The most natural explanation is that the oscillation arises from the rotation of an accreting magnetic (B_S>~10⁵G) white dwarf. This discovery constrains the outburst mechanisms, since the oscillation emission region near the surface of the white dwarf was visible during the outburst.