A search for rapid photometric variability in symbiotic binaries

We report on our survey for rapid (time-scale of minutes) photometric variability in symbiotic binaries. These binaries are becoming an increasingly important place to study accretion on to white dwarfs, since they are candidate Type Ia supernova progenitors. Unlike in most cataclysmic variables, the white dwarfs in symbiotics typically accrete from a wind, at rates greater than or equal to 10^-9M_solaryr^-1. In order to elucidate the differences between symbiotics and other white dwarf accretors, as well as to search for magnetism in symbiotic white dwarfs, we have studied 35 symbiotic binaries via differential optical photometry. Included in our sample are all but one of the symbiotics from the lists of Kenyon and Downes & Keyes with published V magnitudes less than 14 and declinations greater than -20°. Our study is the most comprehensive to date of rapid variability in symbiotic binaries. We have found one magnetic accretor, Z And, previously reported by Sokoloski & Bildsten. In four systems (EG And, BX Mon, CM Aql and BF Cyg), some evidence for flickering at a low level (roughly 10mmag) is seen for the first time. These detections are, however, marginal. For 25 systems, we place tight upper limits on both aperiodic variability (flickering) and periodic variability, highlighting a major difference between symbiotics and cataclysmic variables. The remaining five of the objects included in our sample (the two recurrent novae RS Oph and T CrB, plus CH Cyg, o Ceti and MWC 560) had previous detections of optical flickering. We discuss our extensive observations of these previously known flickering systems in a separate paper. Five new variable stars were discovered serendipitously in the fields of the survey objects, and the observations of these stars are also presented elsewhere. We discuss the impact of our results on the `standard' picture of wind-fed accretion, and speculate on the possibility that light from quasi-steady nuclear burning on the surface of the white dwarf hides the fluctuating emission from accretion.