SIMBAD references

Average V-band brightnesses over multiple outbursting and standstill intervals are computed for the light curves of five Z Cam-type dwarf novae: RX And, Z Cam, SY Cnc, AH Her, and HX Peg. Both CCD photometry from 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{dot} for operation of the thermal limit cycle that is thought to be responsible for dwarf nova outbursts. However, the unusual standstills or hesitations observed in SY Cnc are sometimes fainter than the mean brightness during outbursting intervals. From several well-observed examples we describe the light curves of Z Cam stars as they enter standstill. During this characteristic damped oscillation the amplitude decreases with a timescale approximately equal to the mean outburst recurrence interval, and the period of the oscillation decreases with P{dot}≃-0.4 as standstill is approached. The exits from standstill usually change abruptly into outbursting mode by emerging from standstill into a decline from outburst.