1
No.113Apr. 2023

Variable Star Bulletin
MASTER OT J055845.55+391533.4: SU UMa star with a dip and long rebrightening
Taichi Kato1 , Hiroshi Itoh2 , Tonny Vanmunster3 , Seiichiro Kiyota4 , Katsuaki Kubodera5 ,Pavol A. Dubovsky6 , Igor Kudzej6 , Tomáš Medulka6 , Filipp D. Romanov7 , 8 , David J. Lane9 , 8
1 Department of Astronomy, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japantkato@kusastro.kyoto-u.ac.jp
2 Variable Star Observers League in Japan (VSOLJ), 1001-105 Nishiterakata, Hachioji, Tokyo 192-0153, Japan
pxb02072@nifty.com
3 Center for Backyard Astrophysics Belgium, Walhostraat 1A, B-3401 Landen, Belgium
tonny.vanmunster@gmail.com
4 VSOLJ, 7-1 Kitahatsutomi, Kamagaya, Chiba 273-0126, Japanskiyotax@gmail.com
5 VSOLJ, 2708-1 Kozu, Odawara, Kanagawa, 256-0812, Japanhfd00771@nifty.com
6 Vihorlat Observatory, Mierova 4, 06601 Humenne, Slovakia
var@kozmos.sk
7 Pobedy street, house 7, ﬂat 60, Yuzhno-Morskoy, Nakhodka, Primorsky Krai 692954, Russia,
and remote observer of Abbey Ridge Observatory9 ,ﬁlipp.romanov.27.04.1997@gmail.com, https://orcid.org/0000-0002-5268-7735
8 American Association of Variable Star Observers (AAVSO)
9 Abbey Ridge Observatory, 45 Abbey Rd, Stillwater Lake, NS, B3Z1R1, Canada,
dave@davelane.ca, https://orcid.org/0000-0002-6097-8719
Received 2023 Apr. 15
Abstract
We analyzed Asteroid Terrestrial-impact Last Alert System (ATLAS), Zwicky Transient Facility (ZTF)
and All-Sky Automated Survey for Supernovae (ASAS-SN) data of MASTER OT J055845.55+391533.4 andfound that this object repeats superoutburst with a dip in the middle of the outburst followed by long andsometimes oscillating rebrightening, just like a WZ Sge-type dwarf nova or an AM CVn-type object. The
mean supercycle was 298(8) d, too short for a WZ Sge star, but with only a few normal outbursts. We also
observed the 2023 February–March superoutburst and established the superhump period of 0.05509(2) d. This
period appears to exclude the possibility of an AM CVn star. Although the 2023 observations could not detectsuperhumps after the dip, the 2014, 2016 and 2021 data seem to suggest that low-amplitude superhumps were
present during the rebrightening phase. We note that a dip during a superoutburst is a feature common to
the unusual SU UMa-type dwarf nova MASTER OT J172758.09+380021.5 during some of its superoutbursts.
These objects may comprise a new class of rebrightening phenomenon in SU UMa-type dwarf novae.

1

Introduction

MASTER OT J055845.55+391533.4 was detected as an optical transient on 2014 February 19 at a magnitude of
14.4 (Yecheistov et al. 2014). The object was found to be already at 13.9 mag on 2014 February 13. Two pastoutbursts had been detected (Yecheistov et al. 2014). This object was conﬁrmed to be an SU UMa star by thedetection of superhumps in 2014 (Kato et al. 2015). The superhump period and supercycle were suggested tobe 0.0563(4) d and 360–450 d, respectively (Kato et al. 2015) [for general information of cataclysmic variables(CVs) and dwarf novae (DNe), see e.g., Warner (1995)]. Kato et al. (2017) observed this object on three nights

2
in 2016 and obtained a period of 0.0581 d. Kato et al. (2017) already reported that the outburst behavior was
rather strange. We have noticed that this object showed a dip and rebrightening in superoutbursts recordedin modern survey data (see below). Furthermore, such superoutburst with a dip and (sometimes complex)
rebrightening occur successively without intervening normal outbursts. Such behavior was very unusual for ahydrogen-rich DN but is frequently seen in helium-rich AM CVn objects [although not very apparent in ﬁgure
2 of Levitan et al. (2015), CP Eri is such an object (see section 4)]. We therefore initiated a time-resolved
photometic campaign during the 2023 February–March superoutburst to see the development of superhumps andthe superoutburst. This object is also known as a variable star ZTF J055845.48+391533.1 (Ofek et al. 2020) and
Gaia DR3 3458275544681382912 (=Gaia16bgq, type CV) (Gaia Collaboration et al. 2022).

2

Data analysis

We used Asteroid Terrestrial-impact Last Alert System (ATLAS: Tonry et al. 2018) forced photometry (Shingles
et al. 2021), Zwicky Transient Facility (ZTF: Masci et al. 2019)1 and All-Sky Automated Survey for Supernovae(ASAS-SN: Shappee et al. 2014) Sky Patrol (Kochanek et al. 2017) data to examine the long-term behavior. The
ASAS-SN positive detections fainter than V =16.3 or g=16.5 were excluded as noises close to the detection limit.
The ASAS-SN data around BJD 2459252 were included even below this limit since the reality of these data wereconﬁrmed by a comparison with the ZTF data. Some unﬁltered snapshot observations reported to VSOLJ andVSNET (Kato et al. 2004) were also included (hereafter CCD).
Time-resolved photometry during the 2023 February–March superoutburst was obtained by VSOLJ members and the VSNET Collaboration using 30cm-class telescopes. We also obtained observations during the 2021
January–February superoutburst. The log of these observations is listed in table 1. Superhumps maxima were determined using the template ﬁtting method introduced in Kato et al. (2009) after correcting zero-point diﬀerences
between the observers and removing outburst trends by locally-weighted polynomial regression (LOWESS: Cleveland 1979). The superhump period was determined using the phase dispersion minimization (PDM: Stellingwerf
1978) method, whose errors were estimated by the methods of Fernie (1989); Kato et al. (2010).

3

Results

3.1

Long-term behavior

Long-term light curves are shown in ﬁgures 1 and 2. The ATLAS data in quiescence were systematically brighter
than the ZTF data. This is probably due to the contamination from a red nearby (physically unrelated) companion Gaia DR3 3458275544681383552 (Gaia BP =18.53 and RP =16.65, Gaia Collaboration et al. 2022). Eightsuperoutbursts were recorded in this interval. Four representative well-observed superoutbursts are also shownin ﬁgure 3 to show more details.
• 2016 January–February (BJD 2457416–2457434, ﬁrst panel in ﬁgure 1). ASAS-SN recorded a dip in the
middle of this outburst.
• 2016 August–September (BJD 2457631–2457646, ﬁrst panel in ﬁgure 1). This outburst was observed by
ASAS-SN with on three nights. There was fading in the middle of the outburst, although observations
before and after it were very sparse. There were also time-resolved observations in the late stage (Katoet al. 2017) and the long duration of the outburst was secure. We consider that this outburst was likely asuperoutburst with a dip.
• 2018 April–May (BJD 2458215–2458236, third panel in ﬁgure 1 and ﬁrst panel in ﬁgure 3). ZTF and
ASAS-SN observations clearly showed the presence of a dip in the middle of this outburst (the ASASSN observation during the dip completely overlapped the ZTF point and is invisible in the ﬁgures). The
duration of the dip appeared to be relatively short.
• 2019 March–April (BJD 2458571–2458596, fourth panel in ﬁgure 1 and second panel in ﬁgure 3). A relatively
long dip was clearly present in ZTF, ATLAS and ASAS-SN observations.
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>.

3

Table 1: Log of time-resolved photometry.
Start∗End∗
Mean mag. Error N † Observer‡
Filter2459254.9039 2459255.1868
14.2540.002 235
KubC
2459256.0870 2459256.1791
14.2230.006 73
Kub
C
2459998.2890 2459998.5102
14.3680.004 200
Van
CV
2459999.0367 2459999.1477
14.4050.006 236
Kis
C
2460001.4188 2460001.5810
14.7480.003 226
Van
CV
2460001.9571 2460002.1224
14.8330.004 371
Kis
C
2460001.9729 2460002.0829
14.6310.008 95
Kub
V
2460002.0046 2460002.1895
14.7130.004 269
Ioh
C
2460002.9033 2460003.1528
14.7880.002 448
Ioh
C
2460003.0090 2460003.1402
14.9310.003 306
Kis
C
2460003.9171 2460004.1300
14.9620.004 325
Ioh
C
2460004.0440 2460004.1377
15.1510.006 121
Kis
C
2460004.2960 2460004.5102
15.2800.005 225
Van
CV
2460006.9644 2460007.1752
15.4000.004 237
Ioh
C
2460007.2965 2460007.4774
2.401
0.003 129Vih
C
2460007.9093 2460008.1102
15.4000.034 66
Ioh
C
2460010.0027 2460010.1063
15.976
0.033 30Ioh
C
2460010.0181 2460010.1208
16.1420.015 127
Kis
C
2460010.9498 2460011.1429
15.5060.007 178
Ioh
C
2460011.9477 2460012.0664
15.3250.022 31
Ioh
C
2460013.9407 2460014.1841
15.5320.013 206
Ioh
C
2460014.4756 2460014.5524
15.6700.007 77
Van
CV
2460014.9528 2460015.1516
16.1670.017 113
Ioh
C
2460015.3553 2460015.5511
16.3280.006 187
Van
CV
2460016.4865 2460016.5698
15.9120.003 103
RFD
CV
2460017.3520 2460017.5518
16.9720.006 175
Van
CV
2460018.3133 2460018.5511
17.2670.006 194
Van
CV
2460019.3032 2460019.3055
17.440
0.078
3
Van
CV
2460020.3226 2460020.3339
4.305
0.006
8
Vih
C
∗
BJD−2400000.
†
Number of observations.
‡
Itoh (Ioh), Kiyota (Kis), Kubodera (Kub), Romanov (RFD), Vanmunster (Van),
Vihorlat team (Vih).

4

Table 2: Superhump maxima of MASTER OT J055845.55+391533.4 (2023).
E
Max∗
Error
O − C†
N‡E
0 59998.3208
0.00070.000350
69
1 59998.3778
0.00080.002239
70
2 59998.4322
0.00070.001533
84
3 59998.4864
0.00100.00063785
13 59999.0432
0.00130.006758
86
14 59999.0911
0.0012−0.0006 103
87
57 60001.4559
0.0004−0.004263 10358 60001.5058
0.0009−0.0093
65 10459 60001.5686
0.0006−0.001645 10567 60002.0086
0.0019−0.0023 167 110
68 60002.0608
0.0009−0.0052 215 111∗
BJD−2400000.
†
Against max = 2459998.3205 + 0.055079E.
‡
Number of points used to determine the maximum.

Max∗
60002.1227
60002.1726
60002.9464
60003.0000
60003.0619
60003.1125
60004.0014
60004.0509
60004.1005
60004.3823
60004.4368

Error
0.00310.00200.00100.00170.00120.00220.00310.00280.00340.00220.0045

O − C†
0.0017−0.0035−0.0008−0.00230.00460.00010.00770.0021−0.0034
0.00310.0025

N‡
120
39
80
102
181
183
72
107
122
54
51

• 2020 May (BJD 2458975–2458981, ﬁrst panel in ﬁgure 2). Only the initial part of the outburst was recorded
due to the seasonal gap in observation. The duration of the outburst, however, was suﬃcient for a superoutburst.
• 2021 January–February (BJD 2459242–2459263, second panel in ﬁgure 2 and third panel in ﬁgure 3). A
relatively long dip was clearly present in ZTF, ATLAS and ASAS-SN observations. This outburst showedsome short-term variations (oscillations) after the dip.
• 2021 September–October (BJD 2459467–2459488, third panel in ﬁgure 2 and fourth panel in ﬁgure 3). A
relatively long dip was clearly present in ZTF, ATLAS and CCD observations. An outburst in ZTF and
ATLAS on 2021 September 2–5 (BJD 2459460–2459463) was apparently a precursor.
• 2023 February–March (BJD 2459997–2460020, fourth panel in ﬁgure 2). There was a dip in the middle ofthe outburst. This is the superoutburst during which we made a time-resolved photometric campaign inthis paper (see ﬁgure 5).
The shortest intervals between the superoutburst was 215 d, 225 d being the second shortest. Assuming that
two superoutbursts were missed during the seasonal gaps in 2017 and 2022, the mean interval of the superoutbursts
was 298(8) d.
Only three normal outbursts were detected: 2017 February 13 (BJD 2457798, second panel in ﬁgure 1), 2018
September 16 (BJD 2458378, fourth panel in ﬁgure 1) and 2022 February 8 (BJD 2459619, third panel in ﬁgure1). These results indicate that normal outbursts are essentially rare in this object.

3.22023 February–March superoutburst

A PDM analysis and the mean proﬁle of superhumps before the dip are shown in ﬁgure 4. The mean period inthis interval was 0.05509(2) d. Superhumps were below the detection limit during the rebrightening phase after
the dip. The times of superhump maxima are listed in table 2. The period derivative (Pdot = Ṗ /P ) was +7(2)× 10−5 (see Kato et al. 2009). The O − C diagram, variation of the superhump amplitudes and the light curve
are shown in ﬁgure 5.

3.3

2021 January–February superoutburst

We also observed this object on two nights during the 2021 January–February superoutburst (second panel
in ﬁgure 2). The observations (BJD 2459254.907–2459256.183) were after the dip. A PDM analysis detectedsuperhumps with a period of 0.05454(12) d (ﬁgure 6).

5

14
16
18
ASN g/V
57250

ZTF g

ZTF r

57300

57350

ATLAS oATLAS c

57450

57500

57400

CCD
57550

14
16
18

57600

57650

57700

57750

57800

57850

57900

14
16
18

57950

58000

58050

58100

58150

58200

58250

58300

14
16
18

58300

58350

58400

58450

58500

58550

58600

58650

Figure 1: Light curve of MASTER OT J055845.55+391533.4 in 2015–2019. ASN refers to ASAS-SN and CCD
refers to unﬁltered snapshot observations.

6

14
16
18

58700

58750

58800

58850

58900

58950

59000

14
16
18

59050

59100

59150

59200

59250

59300

59350

14
16
18

59400

59450

59500

59550

59600

59650

59700

59750

14
16
18

59800

Figure 2:
ﬁgure 1.

59850

59900

59950

60000

60050

60100

Light curve of MASTER OT J055845.55+391533.4 in 2019–2023. The symbols are the same as in

7

14
16
18

58190

58200

58210

58220

58230

58240

58250

58260

14
16
18

58550

58560

58570

58580

58590

58600

58610

58620

59220

59230

59240

59250

59260

59270

59280

59290

14
16
18

14
16
18

59440

59450

59460

59470

59480

59490

59500

59510

Figure 3: Well-observed superoutbursts of MASTER OT J055845.55+391533.4. The symbols are the same as
in ﬁgure 1. All superoutbursts had a dip in the middle and subsequent long rebrightening.

8

θ
1.00

0.95

0.90

P=0.05509
0.052

0.054

0.056

0.058

(d)
−0.06

−0.04

−0.02

0.00

0.02

0.04

0.06

−0.5

0.0

0.5

1.0

1.5

Figure 4: Mean superhump proﬁle of MASTER OT J055845.55+391533.4 during the 2023 February–Marchsuperoutburst. The data before the dip (before BJD 2460005) were used. (Upper): PDM analysis. The bootstrap
result using randomly contain 50% of observations is shown as a form of 90% conﬁdence intervals in the resultant
θ statistics. (Lower): Phase plot.

9

0.02

−50

0

50

100

150

200

250

300

350

400

0.01

0.00

−0.01

0.15

0.10

0.05

0.00
14

16

18

0

5

10

15

20

Figure 5: 2023 February–March superoutburst of MASTER OT J055845.55+391533.4. (Upper:) O−C variation.
The ephemeris of BJD(max) = 2459998.3205+0.055079E was used. The data are in table 2. (Middle:) Superhump
amplitude. (Lower:) Light curve. The data were binned to 0.018 d (black ﬁlled circles). Other symbols are thesame as in ﬁgure 1.

10

θ
1.00

0.95

0.90

0.85

P=0.05454

0.80

0.051

0.052

0.053

0.054

0.055

0.056

0.057

0.058

(d)

−0.05

0.00

0.05

−0.5

0.0

0.5

1.0

1.5

Figure 6: Mean superhump proﬁle of MASTER OT J055845.55+391533.4 during the 2021 January–February
superoutburst. The two-night data were obtained after the dip. (Upper): PDM analysis. (Lower): Phase plot.

11

4Discussion

The recorded superhumps in 2023 were identiﬁed as stage B (for superhump stages, see Kato et al. 2009) basedon the proﬁle, decreasing amplitude and the epoch when superhumps were observed. The observed Pdot = +7(2)× 10−5 was not unusual for this short superhump period (Kato et al. 2009).
In Kato et al. (2015), the 2014 superoutburst was observed already at least 15 d after the start of theoutburst and the recorded superhumps were suggested to be stage C. The 2023 data, however, showed no signof stage C nor evidence for superhumps in the late stage of the superoutburst. Although the period 0.0563(4) dwas slightly diﬀerent from the current measurement, it can be consistent with the present result considering thatthe amplitude of superhumps was very small in the 2014 data and that the period was based on single-night
data by a single observer. The 2016 observations (Kato et al. 2017) were also obtained near the end of thesuperoutburst. The period (0.0581 d) appears to be a one-day alias of the present result. Although the 2021
observations (subsection 3.3) were limited, superhumps after the dip appeared to have been detected. These
results suggest that low-amplitude superhumps were present during the rebrightening phase, which were below
the detection limit of the 2023 observations. The superhump period during the rebrightening phase, however,has not yet been well-determined.
The presence of a dip in the middle of a superoutburst is a strange feature in this object. As stated in section 1,
AM CVn stars generally show this feature. We present a light curve of CP Eri in ﬁgure 7 for a comparison. All
well-observed superoutbursts have a dip in the middle and the number of normal outbursts is relatively low
despite the short supercycle. The observed superhump period of MASTER OT J055845.55+391533.4, however,is strongly against the possibility of an AM CVn-type object.
There is another known example of very complex superoutbursts in the very unusual object MASTER OTJ172758.09+380021.5, which has a short superhump period of 0.05829 d and a short supercycle of 50–100 d(Pavlenko et al. 2021). The complex superoutburst recorded in 2022 is shown in ﬁgure 8 (see also ﬁgure 5 inPavlenko et al. 2021 for the past ones). After a relatively long dip and short rebrightening, this object entered long
rebrightening. The behavior is similar to MASTER OT J055845.55+391533.4 in that the superoutburst showeda dip (but more structured) in the middle of a superoutburst. The short orbital period and supercycle arealso similar to MASTER OT J055845.55+391533.4. In the case of MASTER OT J172758.09+380021.5, optical
spectra conﬁrmed its hydrogen-rich nature (Thorstensen et al. 2016; Pavlenko et al. 2021). The diﬀerences between
MASTER OT J055845.55+391533.4 and MASTER OT J172758.09+380021.5 are that the dip in superoutburst
is highly reproducible in the former while it is not in the latter and that the supercycle is a few times shorter inthe latter.
Such a dip during a superoutburst and long rebrightening are usually seen in WZ Sge stars (Kato 2015), but
not in ordinary SU UMa stars. In WZ Sge stars, a cooling wave in the accretion disk somehow occurs during a
superoutburst and the remaining matter in the disk is considered to be responsible for long or repeated rebrightening (Meyer and Meyer-Hofmeister 2015). Low mass-ratios (q) such as in WZ Sge stars (and some ER UMa
stars such as RZ LMi) have been proposed to be a cause of such premature quenching of a superoutburst (Hellier
2001; Osaki 1995). The q value of MASTER OT J172758.09+380021.5 was estimated to be 0.08 (Pavlenko et al.
2021), which seems to be consistent with this interpretation. There have been no indication of WZ Sge-type
behavior neither in MASTER OT J055845.55+391533.4 nor MASTER OT J172758.09+380021.5, and these objects may comprise a new class of rebrightening phenomenon in SU UMa-type dwarf novae. Further observationsof MASTER OT J055845.55+391533.4 are needed to establish the orbital period, and hopefully the period ofstage A to estimate q (Kato and Osaki 2013; Kato 2022), and spectrocopy is needed to estimate the chemicalcomposition. Since superoutbursts of MASTER OT J055845.55+391533.4 are not rare, planned observations fora future superoutburst are requested.

Acknowledgements
This work was supported by JSPS KAKENHI Grant Number 21K03616. The authors are grateful to the ATLAS,ZTF and ASAS-SN teams for making their data available to the public. We are grateful to VSOLJ and VSNET
observers (particularly Yutaka Maeda, Mitsutaka Hiraga and Masayuki Moriyama) who reported snapshot CCD
photometry of MASTER OT J055845.55+391533.4. We are also grateful to Naoto Kojiguchi for helping downloading the ZTF data and to Junpei Ito and Katsuki Muraoka for converting the data reported to the VSNET
Collaboration.
This work has made use of data from the Asteroid Terrestrial-impact Last Alert System (ATLAS) project.

12

16

18

20
58650

58700

58750

58800

58850

58900

16

18

20
59050

59100

59150

59200

59250

16

18

20
59400

59450

59500

59550

59600

59650

16

18

20
ZTF r
59750

59800

59850

ZTF g
59900

ATLAS oATLAS c

59950

60000

Figure 7: Light curve of CP Eri. The presence of a dip in the middle of a superoutburst is the feature common
to MASTER OT J055845.55+391533.4.

13

14

16

18
59700

59710

59720

59730

59740

59750

Figure 8: Complex superoutburst in MASTER OT J172758.09+380021.5 recorded in 2022. The symbols are
the same as in ﬁgure 7.

The ATLAS project is primarily funded to search for near earth asteroids through NASA grants NN12AR55G,
80NSSC18K0284, and 80NSSC18K1575; byproducts of the NEO search include images and catalogs from thesurvey area. This work was partially funded by Kepler/K2 grant J1944/80NSSC19K0112 and HST GO-15889,
and STFC grants ST/T000198/1 and ST/S006109/1. The ATLAS science products have been made possible
through the contributions of the University of Hawaii Institute for Astronomy, the Queen’s University Belfast,
the Space Telescope Science Institute, the South African Astronomical Observatory, and The Millennium Instituteof Astrophysics (MAS), Chile.
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
AM CVn, CP Eri, RZ LMi, WZ Sge, SU UMa, ER UMa, Gaia DR3 3458275544681382912,
Gaia DR3 3458275544681383552, Gaia16bgq, MASTER OT J055845.55+391533.4,
MASTER OT J172758.09+380021.5, ZTF J055845.48+391533.1

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