On the absence of nova shells

We present our wide field Halpha+N[II] observations of 15 cataclysmic variables to search for remnant nova shells. Such shells have been found around other cataclysmic variables that were hitherto not known as novae. Our candidates were selected as objects in the period regime of high-mass transfer systems that experience - at least occasionally - low mass transfer rates. The fact that we find no indication of a nova shell in any of these systems allows us to set a lower limit of 13000 years to the recurrence time of these objects.