A narrowband imaging survey of symbiotic stars

Narrowband images of 51 symbiotic stars were obtained at the ESO and ORM ob servatories to search for resolved optical nebulae. Ionized nebulae much la rger than previously known are found around CH Cyg, HM Sge and V1016 Cyg, a nd they will be discussed in detail in forthcoming papers. A nebula with a deconvolved size between 0".9 and 1".5 is found around the symbiotic Mira H 1-36. The radio sources located few arcminutes aside of R Aqr (Hollis et a l. 1987), which were suggested to be the remnant of a prehistoric eruption of the system, are found to be background galaxies. We also present a bibliographical compilation, updated to October 1998, of all the extended nebulae around symbiotic stars detected at optical and rad io wavelengths, as well as a list of optical non-detections. The statistics of occurrence of these large ionized nebulae among symbiotic stars is discussed. Extended ionized nebulae appear to be a common compone nt of the D-type symbiotics, and we infer that they are formed by the Mira wind and/or high velocity winds ejected by the hot component during outburs ts. On the contrary, very few nebulae are detected around the systems containin g normal red giants'.