[vsnet-alert 24712] DDE 72 anomalous outburst pattern

[Denis Denisenko d.v.denisenko@gmail.com via vsnet-alert]

This dwarf nova in Andromeda (Gaia DR2 position: 00 21 46.00 +40 04
11.9) was discovered in March 2017. In September 2018 Taichi Kato
classified it as a Z Cam-type variable based on the ASAS-SN light
curve, see [vsnet-chat 8234].

Over the last three months the star is showing a very interesting
outburst pattern in ZTF data (Masci et al., 2019). See
https://lasair.roe.ac.uk/object/ZTF17aaaenhs/ (click "Apparent
Magnitude" radio button and select the desired rectangular area to
zoom on the light curve; double click to return to the whole dataset).

I have plotted the recent ZTF light curve for your convenience here:
http://scan.sai.msu.ru/~denis/DDE72-ZTF-LC-2020.gif

At the first glance this looks like the interchanging superoutbursts
and normal outbursts (1SO-1NO-1SO-1NO and so on). But a pattern like
this is not normally observed in SU UMa-type stars (please correct me
if there are such examples). Yet there are no quiescent states between
the outbursts.

On the other hand, a pattern like this is not normally observed in Z
Cam-type stars either. Z Cam outbursts are saw-toothed with sharp
peaks of the nearly same rise and fall time. During the long outbursts
DDE 72 is spending about 10 days above the normal outburst peak,
fading at 0.1 mag/day rate.

This behavior is very similar to that of anomalous Z Cam-type star IW
And in July 2010 - March 2011, as can be seen in AAVSO database. IW
And has shown at least three cycles of "one long-one short" outbursts
in August-September 2010 followed by a standstill in October 2010 and
then again at least four "one long-one short" cycles in November 2010
- January 2011.

It is very interesting to find out how long the current behavior of
DDE 72 will persist. ZTF will hopefully provide nearly nightly
snapshots. But it would be very desirable to see if there are
short-period variations in the long outburst that has started just
now. The orbital period of DDE 72 is unknown. Now is the best season
to observe DDE 72 - it will be in opposition with the Sun on Sep.
28th. The star is one magnitude fainter than IW And.

Resume: a very interesting object in an unusual state. Also a good
food for thought in modeling the "quantum states" of cataclysmic
variables. What combination of parameters is leading to this temporary
stable pattern of long-short-long-short outbursts, if not superhumps?

Denis Denisenko