Three Z Camelopardalis-type dwarf novae exhibiting IW Andromedae-type phenomenon

摘要

I found that V507?Cyg, IM?Eri, and FY?Vul are Z?Cam-type dwarf novae and they showed sequences of standstill terminated by brightening, in contrast to fading as ordinary Z?Cam stars do, followed by damping oscillation. These sequences are characteristic to IW?And-type objects (also known as anomalous Z?Cam stars). New additions to the IW?And-type objects suggest that the IW?And-type phenomenon is more prevalent among Z?Cam stars. I suspect that the regularity of the pattern of the IW?And-type phenomenon suggests a previously unknown type of limit-cycle oscillation, and I suggest that the standstill in these objects is somehow maintained in the inner part of the disk and that the thermal instability starting from the outer part of the disk terminates the standstill to complete the cycle.