ARE Z-CAMELOPARDALIS-TYPE DWARF NOVAE BRIGHTER AT STANDSTILL

Average V-band brightnesses over multiple outbursting and standstill i ntervals are computed for the light curves of five Z Cam-type dwarf no vae: RX And, Z Cam, SY Cnc, AH Her, and HX Peg. Both CCD photometry fr om RoboScope and AAVSO visual estimates are used in the analysis, with consistent results from the two databases for the stars in common. In Four of the five stars, the standstills are as bright as or brighter than the mean V-band brightness during outbursting intervals. This is consistent with Z Cam stars being near the upper stability boundary in (M) over dot for operation of the thermal limit cycle that is thought to be responsible for dwarf nova outbursts. However the unusual stand stills or hesitations observed in SY Cnc are sometimes fainter than th e mean brightness during outbursting intervals. From several well-obse rved examples we describe the light curves of Z Cam stars as they ente r standstill. During this characteristic damped oscillation the amplit ude decreases with a timescale approximately equal to the mean outburs t recurrence interval, and the period of the oscillation decreases wit h (P) over dot = -0.4 as standstill is approached. The exits from stan dstill usually change abruptly into outbursting mode by emerging from standstill into a decline from outburst.