1
No.95

Mar. 2022

ISSN 0917-2211

Variable Star Bulletin
V476 Cyg (Nova Cyg 1920) is currently a dwarf nova —ﬁrst such an object in 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. 11

Abstract
V476 Cyg (Nova Cyg 1920) is a bright, fast nova reaching a photographic magnitude of 2.0. Using the
Zwicky Transient Facility (ZTF) public database, I found that this nova is currently a dwarf nova with acycle length of ∼24 d. Compared to other classical novae currently in dwarf nova-type states, outbursts of
V476 Cyg are rapidly rising and short with durations of a few days. Based on the AAVSO observations, thisnova was probably already in the dwarf nova-type phase in 2016, 96 years after the nova eruption. I founda possible orbital period of 0.1018002(6) d using the ZTF data, which would place the object in the period
gap. This supposed short orbital period appears to explain the features and faint absolute magnitudes of theobserved dwarf nova outbursts. If this period is conﬁrmed, V476 Cyg is a classical nova with the shortest
orbital period with distinct dwarf nova outbursts and in which a nova eruption was recorded in the modern
era. I also compared with the outburst properties with V446 Her (Nova Her 1960), which currently shows SS
Cyg-type outbursts. The transition to the dwarf nova-phase in V476 Cyg occurred much earlier (∼100 yr)
than what has been supposed (∼1000 yr) for classical novae below the period gap. V476 Cyg would not only
provide an ideal laboratory of the behavior of an irradiated accretion disk in which tidal instability is expected
to work, but also an ideal laboratory of the eﬀect of a massive white dwarf on dwarf nova outbursts.

1

Introduction

V476 Cyg was discovered as a bright Galactic nova by Denning (1920). The visual peak magnitude by Denning(1920) was 2.2 on 1920 August 23. Photographic observations showed relatively slow rise from 7.0 mag to thepeak (2.0 mag) which took place in 7 d (Campbell 1932). This nova was a fast nova with t2 =16.5 d (Duerbeck1987) or t2 =6 d (Strope et al. 2010). The nova was classiﬁed as a D-class one with a weak dust dip in the light
curve by Strope et al. (2010). Leslie Peltier described that the post-nova could be sometimes glimpsed in his
autobiographical Starlight Nights (Peltier 1965)1 . He indeed followed this nova since its maximum in 1920 (whenhe was at an age of 20) and saw it around 16 mag or slightly below it between 1961 and 1972 according to
the AAVSO International Database2 . Ringwald et al. (1996a) reported a low-resolution spectrum at V =17.33.
Shara et al. (1986) reported a magnitude of V ∼18.7 and Ringwald et al. (1996a) suspected that either there
were signiﬁcant ﬂux errors in Shara et al. (1986) or the nova was variable on a short time-scale. Ringwald et al.
(1996a) discussed that wiggles in the spectrum of V476 Cyg might be a signature of a dwarf nova.

1 I read this story in the book translated to Japanese (Peltier 1985).
2 <http://www.aavso.org/data-download>.

22

V476 Cyg as a dwarf nova

Using the Zwicky Transient Facility (ZTF: Masci et al. 2019) public data3 , I found that this object is currently
a dwarf nova (T. Kato on 2020 March 5, vsnet-chat 84574 ) [for general information of cataclysmic variables anddwarf novae, see e.g. Warner (1995)]. Here I report on this object using the ZTF data up to the end of 2021. The
light curve is shown in ﬁgure 1. I must note, however, neither all outbursts were detected nor all outbursts weredetected at their peaks by ZTF. The quiescent brightness varied relatively strongly, and the object was brightin 2019 August–September (BJD 2458700–2458760). During this bright phase, there was a outburst startingon BJD 2458718 (2019 August 22; ﬁgure 2), which had a shoulder [or referred to as an embedded precursor by
Cannizzo (2012)] and the peak brightness (r ∼16.5 and g ∼16.6) was brighter than the other outbursts. There
were equally bright outbursts in 2018 September, peaking on BJD 2458386 (ﬁrst panel of ﬁgure 1) and in 2020
June, peaking on BJD 2459010 (third panel of 1). The former outburst apparently had a shoulder as in the 2019
August one.
The color was g − r=+0.1 at outburst peak, while it was redder (g − r ∼ +0.5) in quiescence. This wasprobably due to the presence of a close, physically unrelated, companion star Gaia EDR3 2089624258068065152
with a Gaia magnitude G=19.07 (Gaia Collaboration et al. 2021).
There were also CCD observations in the AAVSO International Database between 2016 and 2019. Shortoutbursts can be recognized by comparing with the ZTF data (ﬁgure 3). The AAVSO observations were unﬁltered
CCD ones obtained by HKEB (K. Hills, UK). At least a few outbursts recorded by ZTF were also recorded by
AAVSO CCD observations. There was a bright outburst on 2017 February 14 (unﬁltered CCD magnitude
15.3). The dwarf nova state should have started before 2016. Although there were some CCD observations withsigniﬁcant variations in 2007, the data were not suﬃcient to identify them as dwarf nova outbursts.

3

Nova in the period gap?

The mean outburst interval derived from the best recorded part (BJD 2459300–2459510) was 24.1(1.4) d. The
durations of most these outbursts were short (2–3 d), suggesting that V476 Cyg has a relatively short orbital
period. There was time-resolved photometry by ZTF on one night (ﬁgure 4). This run suggests a period of∼0.10 d. With the help of this candidate period, I analyzed the ZTF data in quiescence (ﬁgure 5) using phasedispersion minimization (PDM: Stellingwerf 1978) analysis after removing the global trends by locally-weighted
polynomial regression (LOWESS: Cleveland 1979). The error was estimated by methods of Fernie (1989) and
Kato et al. (2010). Although this period looks like the orbital period, it might come from the physically unrelatedcompanion star and needs to be conﬁrmed by further observations. If this period is the orbital period of V476
Cyg, this object is in the period gap. This period appears to be consistent with the outburst behavior mostly
showing short outbursts. The brightest dwarf nova outburst in the ZTF data had MV =+5.5 using AV =0.7
(Schaefer 2018) and the Gaia parallax (Gaia Collaboration et al. 2021). This is relatively faint among dwarf
novae (see e.g. Warner 1987) and appears to be consistent with a short orbital period. Kato (2022) showed that
WZ Sge stars start showing superhumps at MV =+5.4. The present result of V476 Cyg is comparable to thisvalue.
The borders of the period gap is somewhat variable depending the authors. I use the range 0.090–0.13 d basedon equation (17) in Knigge et al. (2011). Well-established novae in the period gap include IM Nor (recurrentnova) (Woudt and Warner 2003; Patterson et al. 2022), V Per (Shafter and Abbott 1989; Shafter and Misselt
2006), QU Vul (Shafter et al. 1995), V597 Pup (Warner and Woudt 2009), and some more borderline or lessestablished cases. None of these object shows dwarf nova-type outbursts.

4

Novae showing dwarf nova outbursts after the eruption

There are well-established classical novae which currently show dwarf nova-type outbursts. I summarized themin table 1. There have been many references for GK Per and I only listed a few of them. It might be worth notingthat Robinson (1975) already reported dwarf nova-like outbursts for V446 Her before the nova eruption. Thisphenomenon may have been similar to the reported case in Mróz et al. (2016). V446 Her currently shows dwarf
3 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>.
4 <http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-chat/8457>.

3

16
ZTF g
ZTF r
17

18

19

58200

58250

58300

58350

58400

58450

58500

58550

58600

58650

58700

58750

58800

58850

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58950

59000

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59100

59150

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59300

59350

59400

59450

59500

59550

16

17

18

19
16

17

18

19
16

17

18

19

Figure 1: ZTF light curve of V476 Cyg.

4

16

17

18

58690 58700 58710 58720 58730 58740 58750 58760

Figure 2: ZTF light curve of V476 Cyg. Enlargement of the bright state in 2019 August–September. The tick
represents a shoulder in the bright outburst. The symbols are the same as in ﬁgure 1.

nova-type outburst typical for an SS Cyg star with long and short outbursts in the ZTF data (in contrast to thestatement in Patterson et al. 2013: ﬁgure 6, see also the light curve in 1994 in Honeycutt et al. 1995). V392 Per
was also a dwarf nova (Richter 1970) before the 2018 nova eruption (e.g. Munari et al. 2020), whose most recent
slowly rising outburst was observed in 2016 February–April [detected by the VSOLJ observer Mitsutaka Hiraga
and the AAVSO observer Carey Chiselbrook (cvnet-outburst message on 2016 February 28)]. BC Cas is currently
in IW And-type state (Kato and Kojiguchi 2020) [see e.g. Simonsen (2011); Kato (2019) for IW And-type stars].
A recent light curve for X Ser is also present in Kimura et al. (2018). A discussion on V1017 Sgr can be also
found in Salazar et al. (2017). The most recent dwarf nova-type outburst occurred in 2007. A outburst of V2109
Oph was detected by the Gaia satellite as Gaia21dza5 . This outburst was a slowly rising one and the orbital
period was suspected to be long (T. Kato, vsnet-alert 261786 ).
BK Lyn was suggested to be the counterpart of the Chinese “guest star” in 101 A.D. (Hertzog 1986;
Patterson et al. 2013) and showed a transient ER UMa-type phase in 2011–2012 (Patterson et al. 2013; Kato et al.
2013, 2014). This state had apparently started as early as in 2005 (Kato et al. 2013). The object is currently innovalike state and no dwarf nova outbursts are observed.
Although WY Sge (nova eruption in 1783) was once considered to be a dwarf nova (Shara et al. 1984),
Naylor et al. (1992); Somers et al. (1996) pointed out that it is just an ordinary old nova. Modern ZTF observations do not show any sign of dwarf nova outbursts contrary to the expectation by the hibernation scenario(Shara et al. 1984, 1986). See also Vogt et al. (2018) for modern observations of WY Sge.
Although Vogt et al. (2018) listed old novae showing low-amplitude outburst (they referred to as stunted
outbursts) in V841 Oph, V728 Sco, V1059 Sgr, V849 Oph, V363 Sgr, HS Pup and V2572 Sgr, the dwarf nova-typenature is not apparent from their light curves for most objects. I included only V728 Sco, which showed recurrent
outbursts similar to dwarf novae by more than 1 mag, in the table.
Three of the objects in the table are long-period systems (orbital periods more than 1 d) and have evolved
secondaries. It is understandable that a considerable fraction of this table is composed of such objects, sincethese objects have a large accretion disk and it is unstable to thermal instability even under mass-transfer rates
typical for ordinary (short-period) novalike systems (Kim et al. 1992). Kim et al. (1992) predicted that outbursts
in such systems are inside-out-type, approximately symmetric ones, which agree with the observations of these
post-novae. This is apparently not the case for V476 Cyg. The outbursts in V476 Cyg rise rapidly and they are
5 <http://gsaweb.ast.cam.ac.uk/alerts/alert/Gaia21dza/>.
6 <http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-alert/26178>.

5

15
16
17
18
19

57500

57550

57600

57650

57700

57750

57800

57850

57900

57950

58000

58050

58100

58150

15
16
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19
15

ZTF g
ZTF r
AAVSO

16
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19

58200

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58300

58350

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58450

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58550

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58850

58900

58950

59000

59050

59100

59150

59200

15
16
17
18
19
15
16
17
18
19

Figure 3: Combined ZTF and AAVSO light curve of V476 Cyg. The AAVSO observations were unﬁltered CCD
ones obtained by HKEB (K. Hills, UK). At least a few outbursts recorded by ZTF were also recorded by AAVSO
CCD observations.

6

17.7
17.8
17.9
18.0
18.1
0.56 0.58 0.60 0.62 0.64 0.66 0.68 0.70 0.72 0.74

Figure 4: Short-term variation recorded in r-band time-resolved photometry by ZTF.

Table 1: Novae showing dwarf nova outbursts after the eruption
ObjectV728 Sco
V606 Aql
GK Per
X Ser
V476 CygBC CasV446 Her
V2109 Oph
V1017 Sgr
BK LynEruption
1862
1899
1901
1903
1920
1929
1960
1969
1919
101?

Orbital Period (d)––1.996803
1.478
0.101800?
–
0.2070–5.786290.07498

References
Vogt et al. (2018)
Kato and Kojiguchi (2021)
Crampton et al. (1986); Bianchini et al. (1986); Šimon (2002)
Thorstensen and Taylor (2000); Šimon (2018)
this paper
Kato and Kojiguchi (2020)
Thorstensen and Taylor (2000); Honeycutt et al. (2011)vsnet-alert 26178
Sekiguchi (1992); Webbink et al. (1987)
Ringwald et al. (1996b); Patterson et al. (2013)

7

θ
1.00

0.98

0.96

0.94

0.92

P=0.10180
0.10160.10170.10180.10190.10200.1021

(d)

−0.1

0.0

0.1

−0.5

0.0

0.5

1.0

1.5

Figure 5: PDM analysis of V476 Cyg using the ZTF data in quiescence. (Upper): PDM analysis. A sharp signal
at 0.1018002(6) d was detected. (Lower): mean proﬁle.

8

15

ZTF g
ZTF r

16
17
18
19
58200

58250

58300

58350

58400

58450

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58700

58750

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58850

58900

58950

59000

59050

59100

59150

59200

59250

59300

59350

59400

59450

59500

59550

15
16
17
18
19

15
16
17
18
19

15
16
17
18
19

Figure 6: ZTF light curve of V446 Her. The current behavior is indistinguishable from that of ordinary SS Cygtype dwarf novae. The BJD scale is the same as in ﬁgure 1 (V476 Cyg). One can easily see the shortness ofoutbursts in V476 Cyg.

9apparently outside-in outbursts. If the suspected orbital period is correct, this behavior is consistent with theshort-period nature. Among the table, the only conﬁrmed short-period object is BK Lyn, whose dwarf nova-typephase was likely a transient phenomenon and the suspected nova eruption occurred nearly 2000 years ago. In thisregard, the case of V476 Cyg with a long-lasting dwarf nova-type phase would be unique. Patterson et al. (2013)
estimated that novae below the period gap show dwarf nova outbursts after the nova eruption when the whitedwarf cools suﬃciently after ∼1000 years. If the suspected orbital period of V476 Cyg is correct, this object
can be an exception. The case of V476 Cyg may reﬂect the rapid evolution (with t2 =16.5 d or 6 d) of the nova
eruption and rapid subsequent cooling.

5

Shoulder or failed superoutburst?

The nature of the shoulder in the dwarf nova-type outburst is not still clear. Cannizzo (2012) considered it tobe similar to precursor outbursts in SU UMa-type superoutbursts. Kato and Hambsch (2021) suggested thatit originates when the disk reaches the tidal truncation radius. In the special case of V363 Lyr, the outburst
accompanied by a shoulder was 0.3–0.4 mag brighter than other outbursts and showed periodic modulations witha period slightly longer than the orbital period (Kato 2021). The nature of this variation is still unclear (Kato2021). Compared to the light curves by Cannizzo (2012), such as that of SS Cyg, the case of V476 Cyg looks
more similar to that of V363 Lyr. It would be worth performing time-resolved photometry during such outbursts
to detect possible periodic signals as in V363 Lyr. Other shorter outbursts in V476 Cyg have variable peakbrightness, although it was more constant at 17.0 mag in the late 2020 to the 2021 seasons (later part of the third
panel and the fourth panel of ﬁgure 1). Considering the suspected orbital period in section 3, these outbursts withshoulders may be analogous to SU UMa-type superoutbursts [a “failed superoutburst” is also known in SU UMastars, during which tidal instability is not suﬃciently strong to produce a full superoutburst (Osaki and Kato
2013)], although the durations were much shorter.
Determination of the orbital period by radial-velocity studies is desired. Considering that many dwarf novae
in the period gap have been identiﬁed as SU UMa stars [e.g. V1006 Cyg (Kato et al. 2016); MN Dra (Nogami et al.
2003; Pavlenko et al. 2010; Bąkowska et al. 2017); NY Ser (Pavlenko et al. 2014; Kato et al. 2019)], superoutbursts may be expected in V476 Cyg. Continued observations and timely time-resolved photometry would clarify
the nature of dwarf nova outbursts in V476 Cyg. Since the object appears to be still declining from the 1920 nova
eruption, this object would provide an ideal laboratory of the behavior of an irradiated accretion disk in whichtidal instability is expected to work. This object would also be an ideal laboratory of the eﬀect of a massive
white dwarf on dwarf nova outbursts.

Acknowledgements
This work was supported by JSPS KAKENHI Grant Number 21K03616. The author is grateful to the ZTF teamfor 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 (Watson et al. 2006), the AAVSOInternational Database 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).

10

List of objects in this paper
V606 Aql, BC Cas, SS Cyg, V476 Cyg, V1006 Cyg, MN Dra, V446 Her, BK Lyn, V363 Lyr, IM Nor, V841
Oph, V849 Oph, V2109 Oph, V Per, GK Per, V392 Per, HS Pup, V597 Pup, V728 Sco, X Ser, NY Ser, WY
Sge, WZ Sge, V363 Sgr, V1017 Sgr, V1059 Sgr, V2572 Sgr, SU UMa, ER UMa, QU Vul, Gaia21dza, Gaia EDR3
2089624258068065152

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