AbstractRecent observations of an apparent soft X-ray halo around the dwarf nova SU UHa have led to speculation that this may well be evidence of the object having undergone a classical nova-like outburst within historical times (Cordova and Mason 1980). By combining the relationship between quiescent X-ray luminosity and speed class for classical novae and the observed X-ray luminosity of SU UMa we derive a distance dependent apparent magnitude at outburst for the object. Distance estimates for SU UMa and absolute magnitude ranges for classical novae then determine the apparent magnitude of an outburst more exactly. From the angular size of the halo and the absolute magnitude - ejection velocity relationship for classical novae we derive the approximate date of any outburst. Comparison with historical records does not reveal any promising candidates. An alternative interpretation of the halo in terms of scattering of soft X-rays from dwarf nova outbursts by interstellar grains is suggested.
AT Cnc: A SECOND DWARF NOVA WITH A CLASSICAL NOVA SHELL