Three Z Cam-Type Dwarf Novae Exhibiting IW And-Type Phenomenon

Abstract

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 in ordinary Z Cam stars, 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 suggests 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 the thermal instability starting from the outer part of the disk terminates the standstill to complete the cycle.

5 pages, 6 figures, accepted for publication in PASJ

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