1No.94

Mar. 2022

ISSN 0917-2211

Variable Star BulletinLong-lasting standstill and fading episode in the IW And star V507 Cyg
Taichi Kato,1 Masayuki Moriyama2tkato@kusastro.kyoto-u.ac.jp
1 Department of Astronomy, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan
2 Variable Star Observers League in Japan (VSOLJ), 290-383, Ogata-cho, Sasebo, Nagasaki 858-0926, Japan

Received 2022 Mar. 08Abstract
We studied the IW And star V507 Cyg using ASAS-SN data, ZTF data and our snapshot photometry.
The star has been found to be in a long standstill in 2020 May–2022 March (and it still continues now). Suchlong-lasting standstills have been found in a few other IW And stars in the past literature. We found that thissystem was systematically fainter by 0.3 mag between 2018 March and 2019 January when it showed dwarf
nova-type variations, including IW And-type states. This is one of the clearest pieces of evidence that thedwarf nova-type state in Z Cam stars (in a broader sense) is associated with a decrease in the mass-transferrate (∼25% for a duration of 300 d in this case). The object also showed IW And-type phenomena in 2019,
when the system was as bright as in the current long standstill. This ﬁnding suggests that the condition
whether the IW And-type phenomenon occurs or not is subtle under the same mass-transfer rate.

V507 Cyg is one of the prototypical IW And-type stars (for references of IW And stars, see Simonsen 2011;
Szkody et al. 2013; Hameury and Lasota 2014; Kato 2019). We report on the detection of a long standstill in2020 May–2022 March. We used observations by the All-Sky Automated Survey for Supernovae (ASAS-SN) Sky
Patrol data (Shappee et al. 2014; Kochanek et al. 2017), the Zwicky Transient Facility (ZTF: Masci et al. 2019)
data1 and unﬁltered snapshot CCD observations by one of the authors (MM).
When Kato (2019) initially reported on this object, this object primarily showed dwarf nova-type variations
sometimes interrupted by short standstills, which had characteristics of IW And stars as follows: (1) Standstills
were terminated by brightening. (2) There were quasi-periodic cycles consisting of a (quasi-)standstill with damping oscillations — brightening which terminates the standstill — a deep dip and returning to a (quasi-)standstill(see ﬁgure 1 around BJD 2457950–2458040 and BJD 2458275–2458330). After this IW And-type state, which
was followed by an ordinary dwarf nova state (at the end of ﬁgure 1 and the beginning of ﬁgure 2), long standstills appeared, which were interrupted by brightening (BJD 2458675, 2458783 and 2458836) followed by a dipand subsequent damping oscillations, which are characteristic to an IW And-type object. After BJD 2458990(2020 May), this object entered a long-lasting standstill, which has continued up to now (ﬁgure 2). It would beinteresting to note that the standstill BJD 2458890–2458950 was terminated by fading as in usual Z Cam stars.
Such long-lasting standstills had been recorded in IM Eri (Kato et al. 2020), BO Cet (Kato et al. 2021) and
ASAS J071404+7004.3 (Kato et al. 2022): all these objects were initially considered as novalike stars because ofthis (Chen et al. 2001; Armstrong et al. 2013; Rodríguez-Gil et al. 2007; Inight et al. 2022). A similar IW Andtype object HO Pup (Kimura et al. 2020; Lee et al. 2021), however, did not show such a long standstill lasting
for a year.
1 The ZTF data can be obtained from IRSA <https://irsa.ipac.caltech.edu/Missions/ztf.html> using the interface <https://irsa.ipac.caltech.edu/docs/program_interface/ztf_api.html> or using a wrapper of the above IRSA API<https://github.com/MickaelRigault/ztfquery>.

2We analyzed the the trend of mean magnitudes (averaged in ﬂux) as we performed for two IW And stars
ST Cha (Kato and Kojiguchi 2021) and ASAS J071404+7004.3 (Kato et al. 2022) using locally-weighted polynomial regression (LOWESS: Cleveland 1979). The result is shown in ﬁgure 3. We should note that the trend
before BJD 2458000 was overestimated (brighter than actual) when the ASAS-SN observations were not deep
enough to record the object at minimum brightness. The trend after BJD 2458000 can be considered as real.
It is apparent that the object was systematically fainter by 0.3 mag between BJD 2458200 and 2458500, when
the object showed dwarf nova-type variations including the IW And-type states. Although it is widely believedthat standstills in Z Cam stars reﬂect the states with higher mass-transfer rates (Meyer and Meyer-Hofmeister1983), the results by observations have not been very clear (Honeycutt et al. 1998; Kato et al. 2019; Kato 2021;
Kato et al. 2022). The present case provides the ﬁrst clear evidence that a dwarf nova-type phase (includingthe IW And state) was associated with a fading episode by 0.3 mag (corresponding to a 25% decrease in themass-transfer rate) lasting for ∼300 d. The 2019 state with standstills interrupted by brightening followed bydamping oscillations (BJD 2458560–2458860) was as bright as the following long standstill. This implies thatoccasional IW And-type phenomena also occurred at the same mass-transfer rate as in the long standstill andthat the condition whether IW And-type phenomena occur or not is subtle.

Corrections
While working on this object, TK noticed that there were scaling errors in ST Cha (Kato and Kojiguchi 2021)
and ASAS J071404+7004.3 (Kato et al. 2022). I here present the corrected ﬁgures 4 and 5.

Acknowledgements
This work was supported by JSPS KAKENHI Grant Number 21K03616. The author is grateful to the ASAS-SN
and ZTF teams for making their data available to the public. We are grateful to Naoto Kojiguchi for helping
downolading the ZTF data.
Based on observations obtained with the Samuel Oschin 48-inch Telescope at the Palomar Observatory as
part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under Grant
No. AST-1440341 and a collaboration including Caltech, IPAC, the Weizmann Institute for Science, the Oskar
Klein Center at Stockholm University, the University of Maryland, the University of Washington, Deutsches
Elektronen-Synchrotron and Humboldt University, Los Alamos National Laboratories, the TANGO Consortiumof Taiwan, the University of Wisconsin at Milwaukee, and Lawrence Berkeley National Laboratories. Operations
are conducted by COO, IPAC, and UW.
The ztfquery code was funded by the European Research Council (ERC) under the European Union’s Horizon
2020 research and innovation programme (grant agreement n◦ 759194 – USNAC, PI: Rigault).

List of objects in this paper
IW And, Z Cam, BO Cet, ST Cha, V507 Cyg, IM Eri, HO Pup, ASAS J071404+7004.3References
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Figure 1: Long-term light curve of V507 Cyg using ASAS-SN and ZTF observations. See ﬁgure 2 for thesymbols. V507 Cyg primarily showed dwarf nova-type variations sometimes interrupted by short standstills
bearing characteristics of IW And stars. This ﬁgure corresponds to ﬁgure 2 in Kato (2019) with subsequent
observations.

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Figure 2: Long-term light curve of V507 Cyg using ASAS-SN, ZTF and our unﬁltered CCD observations (C).
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Figure 4: (Upper): ASAS-SN g-band light curve of ST Cha. (Lower): Trends determined by LOWESS. Horizontalmarks represent IW And-type states. (Correction of Kato and Kojiguchi 2021)

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Figure 5: (Upper): ASAS-SN light curve of ASAS J071404+7004.3. (Lower): Trend determined by LOWESS. A
smoothing parameter of f =0.05 was used. (Correction of Kato et al. 2022)

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VSOLJ
c/o Keiichi Saijo National Science Museum, Ueno-Park, Tokyo Japan
Editor Seiichiro Kiyota
e-mail: skiyotax@gmail.com