1
No.96

Mar. 2022

ISSN 0917-2211

Variable Star Bulletin
WFI J161953.3+031909: eclipsing ER UMa-type and Z Cam-type starin the period gap
Taichi Kato1
tkato@kusastro.kyoto-u.ac.jp
1 Department of Astronomy, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan

Received 2022 Mar. 13

Abstract
WFI J161953.3+031909 was considered to be an eclipsing novalike object in the period gap. Using the
Zwicky Transient Facility (ZTF) public database, I found that this object is the ﬁrst eclipsing Z Cam star in
the period gap, and is also most likely an ER UMa star with supercycles of 60–80 d. The longest outburst(most likely a superoutburst) comprised 35–46% of the supercycle in the extreme case. If superhumps are
conﬁrmed, this becomes the second object showing both ER UMa and Z Cam states after NY Ser. These
objects have anomalously high mass-transfer rates despite that they are in the period gap. I reﬁned theorbital period to be 0.099419808(8) d. We can expect to learn from WFI J161953.3+031909 using eclipses
what is actually happening in the ER UMa-type and Z Cam-type disk. I also provide a revised classiﬁcationof an eclipsing IW And star for BMAM-V383 = IPHAS J200822.55+300341.6 by detecting an IW And-type
standstill.

1

Introduction

WFI J161953.3+031909 was discovered by Rau et al. (2006) during a wide-ﬁeld search for orphan afterglows of
gamma-ray bursts (GRBs). The object showed sudden brightening by 2.4 mag between 1999 June 14 and June
19. Although Rau et al. (2006) described that the object decayed over 50–90 d before returning to quiescence,
this ﬁgure was based on very sparse observations (see their ﬁgure 5). Rau et al. (2006) suggested it to be dwarf
nova. Based on detections of 9 X-ray photons with ROSAT, they derived a high LX /Lopt =0.6 and was considered
to be consistent with an SU UMa star referring to Verbunt et al. (1997). Rau et al. (2006) derived a distance oforder of a few hundred parsec based on this X-ray observation.
Rau et al. (2007) studied this object in more detail in 2006 and found it to be an eclipsing binary with aperiod of 0.099041(9) d. Rau et al. (2007) also detected strong Balmer and He I emission lines. Rau et al. (2007)
also attributed a broad feature as a Bowen blend, although there was no feature of He II emission, which should
be present when the Bowen blend is present. Rau et al. (2007) interpreted that WFI J161953.3+031909 has a
low mass-transfer rate expected for an object in the period gap based on the absence of an orbital hump or theasymmetry in the eclipse. Based on this interpretation, Rau et al. (2007) concluded that the “outburst” detected
in 1999 (Rau et al. 2006) represented the mean out-of-eclipse brightness rather than a dwarf nova-type outburst
and that the faint (R=19.9) observations corresponded to eclipses. Rau et al. (2007) concluded that there was
no evidence for a dwarf nova outburst in this system and corrected LX /Lopt to be ∼0.1 using the out-of-eclipse
brightness rather than what had been considered to be quiescence.
Based on the classiﬁcation by Rau et al. (2007), the AAVSO Variable Star Index (Watson et al. 2006) classiﬁed this object to be an eclipsing novalike object (at the time of this writing on 2022 March 13), although it

2
was apparently inconsistent with the low mass-transfer rate as stated by Rau et al. (2007). This object was also
detected as a transient Gaia19cwd1 by the Gaia Photometric Science Alerts Team. The light curve on the pageof Gaia19cwd showed scattered magnitudes between 17.0 and 20.0, but the type of variability is not apparent
due to the sparse coverage

2

ZTF light curve

Using the Zwicky Transient Facility (ZTF: Masci et al. 2019) public data2 , I found that this object was ina standstill at least between 2021 February and September (T. Kato on 2022 March 13, vsnet-chat 90143 ).
Considering that this object is an eclipsing cataclysmic variable in the period gap, this ﬁnding is surprising. I
here analyze the data in more details.
The entire ZTF light curve is shown in ﬁgure 1. The object was initially in dwarf nova-type state (in 2018–2020). After BJD 2459260 (2021 February), it entered a well-deﬁned, long standstill. The object is now conﬁrmed
to be a Z Cam star [for general information of cataclysmic variables and dwarf novae, see e.g. Warner (1995)] in
the period gap. There is only another known Z Cam star in the period gap (NY Ser, Kato et al. 2019), which
will be discussed later. WFI J161953.3+031909 is the ﬁrst eclipsing Z Cam star in the period gap. There is(yet) no indication of the IW And-type phenomenon in this object [see e.g. Simonsen (2011); Kato (2019) for
IW And-type stars].

3

Orbital period

Due to the presence of both dwarf nova-type variations and eclipses, and due to the sparse sampling, period
analysis of the ZTF data is rather diﬃcult. I smoothed g, r and i observations separately by locally-weighted
polynomial regression (LOWESS: Cleveland 1979) using global smoothing parameters of f =0.05 to reduce theeﬀect of dwarf nova outbursts. I then combined them into a single data set and performed phase dispersion
minimization (PDM: Stellingwerf 1978) analysis. The upper panel of ﬁgure 2 shows the result of the PDM
analysis. There was no signal other the indicated one around the period by Rau et al. (2007). The orbital period
was reﬁned using the Markov-Chain Monte Carlo (MCMC)-based method introduced in Kato et al. (2010). The
resultant ephemeris is
Min(BJD) = 2458790.81347(4) + 0.099419808(8)E.
(1)
The averaged orbital light curve based on this ephemeris is shown in the lower panel of ﬁgure 2. Note that
this light curve contains observations in all states (dwarf nova outbursts, quiescence and standstill). The result,however, is very similar to the one in Rau et al. (2007) without a pronounced orbital hump.
Since Rau et al. (2007) reported no variations outside the eclipses on seven epochs in 2006 May, it was likely
Rau et al. (2007) observed the object during a standstill similar to the one in 2021. The brightness (R=17.4–17.8)
outside the eclipses also supports this idea. I cannot, however, perfectly exclude a possibility that they observed
the object during the late phase of a superoutburst when superhumps became less apparent. In either case, theobject was observed when the accretion disk was hot. This explains why there was no pronounced orbital hump,on the contrary to the explanation by Rau et al. (2007) considering a low mass-transfer rate. By looking backusing the ephemeris (1), the epoch listed in Rau et al. (2007) was oﬀset by ∼0.17 phase. I did not attempt to link
the ephemerides since the orbital period in this study was derived from observation far from ideal to determine
the period and the actual error may be much larger than the nominal one shown in equation (1).

4WFI J161953.3+031909 as a likely ER UMa star

During the dwarf nova-type state in 2018–2020, WFI J161953.3+031909 showed long outbursts, which are marked
with vertical ticks in ﬁgure 1. Considering the short orbital period, these long outbursts must have been superoutbursts of an SU UMa star. The longest outburst (the ﬁrst one in the second panel of ﬁgure 1) lasted at least
1 <http://gsaweb.ast.cam.ac.uk/alerts/alert/Gaia19cwd/>.
2 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>.
3 <http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-chat/8457>.

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16
ZTF g
ZTF r
ZTF i

18

20

58200

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58300

58350

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58450

58500

16

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16

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58950

59000

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59100

59150

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59200

59250

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59350

59400

59450

59500

Figure 1: ZTF light curve of WFI J161953.3+031909. The object was initially in dwarf nova-type state. After
BJD 2459260, it entered a well-deﬁned, long standstill. The sporadic excursions to fainter magnitudes represent
eclipses. The vertical ticks in the upper three panels represent likely superoutbursts.

4

θ

1.00

0.95

0.90

P=0.09942

0.85

0.099380

0.099400

0.099420

0.099440

0.099460

(d)
−1

0

1

2

−0.5

0.0

0.5

1.0

1.5

Figure 2: PDM analysis of WFI J161953.3+031909 using the ZTF data. (Upper): PDM analysis. (Lower): mean
proﬁle. The epoch and period were from the MCMC analysis.

528 d, and possibly 37 d. The intervals of successive superoutbursts in the second panel of ﬁgure 1 were 80 dand 65 d. The interval of the successive superoutburst in the third panel of ﬁgure 1 was 60 d. The shortness ofthe supercycle (interval between successive superoutbursts) and the long duration of superoutbursts (comprising
35–46% of the supercycle in the extreme case) are perfectly compatible with the traditional ER UMa-type classiﬁcation [see Kato and Kunjaya 1995; Robertson et al. 1995; Patterson et al. 1995; for a review, see Kato et al.
1999]. Although BMAM-V383 = IPHAS J200822.55+300341.6 (Barentsen et al. 2014) is listed as an eclipsing
ER UMa star in the AAVSO VSX (at the time of present writing), the slow fading rates from outbursts (ﬁgure 3;
see also the light curve of the SS Cyg star CY Lyr in Kato et al. 2014) are incompatible with the ER UMa-typeclassiﬁcation. This object showed a standstill in 2019 July–September with brightening at the end (ﬁgure 3) andit should be re-classiﬁed as an IW And star. Although its orbital period is not yet known, the ﬁgure4 presented
by Mariusz Bajer apparently shows a period longer than the period gap, compatible with an IW And star, but
not with an ER UMa star. V4140 Sgr is the only known eclipsing ER UMa-like star (Kato et al. 2018). If
superhumps are conﬁrmed during superoutbursts of WFI J161953.3+031909, this will become the second case of
eclipsing ER UMa stars.
If superhumps are conﬁrmed (still not yet, but are very likely), WFI J161953.3+031909 is the third object
showing both characteristics of Z Cam and SU UMa stars. The others are NY Ser in the period gap (Kato et al.
2019) and BO Cet, which is also an IW And star, above the period gap (Kato et al. 2021). NY Ser is particularly
similar to WFI J161953.3+031909 in that both show frequent superoutbursts, which is a consequence of a highmass-transfer rate despite that they are in the period gap. At least two standstills in NY Ser were terminatedby superoutbursts (Kato et al. 2019), which conﬁrmed that the radius of an accretion disk can increase during
standstills and this ﬁnding led to a working hypothesis that the IW And-type phenomenon is a manifestationof the increase in the disk radius during standstills (Kato et al. 2019; Kimura et al. 2020; M. Shibata et al. in
preparation). In the case of WFI J161953.3+031909, we can directly determine the evolution of the structureof the disk by using eclipses. We can expect to learn from WFI J161953.3+031909 what is actually happening
in the ER UMa-type and Z Cam-type disk, and potentially about the IW And-type disk if the star exhibits the
IW And-type phenomenon in future.

Acknowledgements
This work was supported by JSPS KAKENHI Grant Number 21K03616. The author is grateful to the ZTF
team for making their data available to the public. We are grateful to Naoto Kojiguchi for helping downloading
the ZTF data. This research has made use of the AAVSO Variable Star Index and NASA’s Astrophysics Data
System.
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).
We acknowledge ESA Gaia, DPAC and the Photometric Science Alerts Team (http://gsaweb.ast.cam.ac.uk/
alerts).

List of objects in this paper
IW And, Z Cam, SS Cyg, CY Lyr, NY Ser, V4140 Sgr, SU UMa, ER UMa, BMAM-V383, IPHAS J200822.55+300341.6,
Gaia19cwd, WFI J161953.3+031909

4 <https://www.aavso.org/vsx_docs/838271/3635/BMAM-V383%20View%20on%20eclipse.PNG>.

6

ZTF g
ZTF r
ZTF i

18

20

22
58200

58250

58300

58350

58400

58450

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Figure 3: ZTF light curve of BMAM-V383 = IPHAS J200822.55+300341.6. The light curve is characterized
by long and short outbursts, which are usually seen in SS Cyg stars. The presence of a standstill terminated bybrightening (IW And-type phenomenon) is apparent in BJD 2459423–2459492.

7

References
Barentsen, G. et al. (2014) The second data release of the INT photometric Hα survey of the northern Galactic
plane (IPHAS DR2). MNRAS 444, 3230
Cleveland, W. S. (1979) Robust locally weighted regression and smoothing scatterplots. J. Amer. Statist. Assoc.
74, 829
Kato, T. (2019) Three Z Cam-type dwarf novae exhibiting IW And-type phenomenon. PASJ 71, 20
Kato, T., Hambsch, F.-J., & Cook, L. M. (2018) Detection of the supercycle in V4140 Sagittarii: First eclipsing
ER Ursae Majoris-like object. PASJ 70, L3
Kato, T. et al. (2014) Survey of period variations of superhumps in SU UMa-type dwarf novae. V: The ﬁfth year
(2012–2013). PASJ 66, 30
Kato, T., & Kunjaya, C. (1995) Discovery of a peculiar SU UMa-type dwarf nova ER Ursae Majoris. PASJ 47,
163
Kato, T. et al. (2010) Survey of Period Variations of Superhumps in SU UMa-Type Dwarf Novae. II. The Second
Year (2009-2010). PASJ 62, 1525
Kato, T., Nogami, D., Baba, H., Masuda, S., Matsumoto, K., & Kunjaya, C. (1999) in Disk Instabilities in Close
Binary Systems, ed. S. Mineshige, & J. C. Wheeler (Tokyo: Universal Academy Press) p. 45
Kato, T. et al. (2019) Discovery of standstills in the SU UMa-type dwarf nova NY Serpentis. PASJ 71, L1
Kato, T. et al. (2021) BO Ceti: Dwarf nova showing both IW And and SU UMa-type features. PASJ 73, 1280
Kimura, M., Osaki, Y., & Kato, T. (2020) KIC 9406652: A laboratory of the tilted disk in cataclysmic variable
stars. PASJ 72, 94
Masci, F.-J. et al. (2019) The Zwicky Transient Facility: Data processing, products, and archive. PASP 131,018003
Patterson, J., Jablonski, F., Koen, C., O’Donoghue, D., & Skillman, D. R. (1995) Superhumps in cataclysmic
binaries. VIII. V1159 Orionis. PASP 107, 1183
Rau, A., Greiner, J., & Schwarz, R. (2006) Constraining the GRB collimation with a survey for orphan afterglows.
A&A 449, 79
Rau, A. et al. (2007) The incidence of dwarf novae in large area transient searches. ApJ 664, 474
Robertson, J. W., Honeycutt, R. K., & Turner, G. W. (1995) RZ Leonis Minoris, PG 0943+521, and V1159
Orionis: Three cataclysmic variables with similar and unusual outburst behavior. PASP 107, 443
Simonsen, M. (2011) The Z CamPaign: Year 1. J. American Assoc. Variable Star Obs. 39, 66
Stellingwerf, R. F. (1978) Period determination using phase dispersion minimization. ApJ 224, 953
Verbunt, F., Bunk, W. H., Ritter, H., & Pfeﬀermann, E. (1997) Cataclysmic variables in the ROSAT PSPC All
Sky Survey. A&A 327, 602
Warner, B. (1995) Cataclysmic Variable Stars (Cambridge: Cambridge University Press)
Watson, C. L., Henden, A. A., & Price, A. (2006) The International Variable Star Index (VSX). Society for
Astronom. Sciences Ann. Symp. 25, 47

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