1
No.120Jul. 2023

Variable Star Bulletin
ASASSN-22ak: La Belle au bois dormant in a hydrogen-depleted dwarf nova?
Taichi Kato1 , Franz-Josef Hambsch2 , 3 , 4 Berto Monard,5 , 6 Rod Stubbings7
1 Department of Astronomy, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japantkato@kusastro.kyoto-u.ac.jp
2 Groupe Européen d’Observations Stellaires (GEOS), 23 Parc de Levesville, 28300 Bailleau l’Evêque, France
hambsch@telenet.be
3 Bundesdeutsche Arbeitsgemeinschaft für Veränderliche Sterne (BAV), Munsterdamm 90, 12169 Berlin,
Germany
4 Vereniging Voor Sterrenkunde (VVS), Oostmeers 122 C, 8000 Brugge, Belgium
5 Bronberg Observatory, Center for Backyard Astrophysics Pretoria, PO Box 11426, Tiegerpoort 0056, South
Africaastroberto13m@gmail.com
6 Kleinkaroo Observatory, Center for Backyard Astrophysics Kleinkaroo, Sint Helena 1B, PO Box 281,
Calitzdorp 6660, South Africa
7 Tetoora Observatory, 2643 Warragul-Korumburra Road, Tetoora Road, Victoria 3821, Australia
stubbo@dcsi.net.au
Received 2023 Jul. 01
Abstract
ASASSN-22ak is a transient discovered by the All-Sky Automated Survey for Supernovae and by Gaia
in 2022 January. Although this object had been in deep quiescence at least for seven years before thisoutburst, it has been showing relatively regular long (35–40 d) outbursts with intervals of 132–188 d after
the 2022 January outburst. This “waking up” phenomenon appears similar to the very unusual (hydrogenrich) WZ Sge star V3101 Cyg. Time-resolved photometry during the 2023 outburst detected low-amplitude(0.05 mag) superhumps with a period of 0.042876(3) d. ASASSN-22ak appears to be very similar to CRTS
J112253.3−111037, which is known to have a very low mass ratio and is considered to be an object evolving
close to AM CVn stars as inferred from the low hydrogen and high helium content. ASASSN-22ak is likely yet
another object having an evolved core and strongly depleted hydrogen in the secondary. The case of ASASSN22ak strengthens the idea that a considerable fraction of AM CVn stars are formed from evolved cataclysmic
variables. Both ASASSN-22ak and V3101 Cyg before the initial outbursts were probably in dormant stateswith low quiescent viscosity or low mass-transfer rates. The current “high” states of ASASSN-22ak and V3101
Cyg may have been induced by radiation during the initial outburst or these objects are simply returning
to ordinary states, either in terms of quiescent viscosity or mass-transfer rates. We also provide updated
superhump period and estimated mass ratio for CRTS J112253.3−111037.

1

Introduction

In the famous fairy tale La belle au bois dormant (the Beauty in the Sleeping Forest or the Sleeping Beauty), a
princess was cursed by an evil fairy to sleep for a hundred years before being awakened by a prince (Perrault 1697).
This tale produced one of the world most famous ballets composed by Pyotr Tchaikovsky (Tchaikovsky 1889)1 .
The similar things appear to have happened in the world of dwarf novae. The giant outburst and subsequent
superoutbursts in V3101 Cyg = TCP J21040470+4631129 (Tampo et al. 2020; Hameury and Lasota 2021) could
1 The reference refers to the earliest publication of this work in the form of a score of Aleksandr Ziloti’s arrangement for solo
piano according to Tchaikovsky’s letter (<https://en.tchaikovsky-research.net/pages/The_Sleeping_Beauty>). The premiere at theMariinsky Theatre was performed in 1890.

2
be a signature of long “dormant” phase before the initial outburst. MASTER OT J030227.28+191754.5 (Tampo
et al. 2023; Kimura et al. 2023) might be another such example. Here, we report on an instance of ASASSN-22ak,
which may be the ﬁrst similar case in a cataclysmic variable (CV) with an evolved core in the secondary.

2

ASASSN-22ak

ASASSN-22ak was discovered as a dwarf nova by the All-Sky Automated Survey for Supernovae (ASAS-SN:
Shappee et al. 2014) at g=15.0 on 2022 January 7.2 The object further brightened and reached the peak ofg=13.2 on 2022 January 8. The object apparently faded rapidly after this (there was a 6-d gap in observation in
ASAS-SN). When the object was observed again on 2022 January 16 by Gaia (=Gaia22afw)3 , the object faded to
G=15.16. This outburst was announced in VSNET (Kato et al. 2004) by Denis Denisenko (vsnet-alert 26518)4 .
According to this, this outburst was also detected by MASTER-OAFA (Lipunov et al. 2010) at 13.8 mag on
2022 January 9. The object underwent another outburst at 15.4 mag on 2022 July 20 detected by one of theauthors (RS) (vsnet-alert 26875)5 and 16.2 mag on 2022 December 18 (by RS, vsnet-alert 27223)6 . After these
two outbursts, the unusual light curve of this object received attention (vsnet-alert 27224).7 The ASAS-SN light
curve suggested that all outbursts were superoutbursts. Although the similarity to V3101 Cyg and the possibilityof an AM CVn star, as judged from the short recurrence time of long outbursts, were discussed, the nature of theobject remained elusive. One of the authors (BM) obtained a single-night run during the 2022 January outburst
and a possible period of 0.044 d was suggested (vsnet-alert 27225).8 This period, however, did not comfortably
ﬁt what is expected for a WZ Sge star and the reality of the period remained to be conﬁrmed. During the 2022
December outburst, one of the authors (FJH) obtained time-resolved photometry, which also suggested a periodof 0.0412 d (vsnet-alert 27243).9 This suggestion of a period, however, remained unconﬁrmed since the object
faded soon after these observations and the amplitudes of the variations were small. The sudden fading of 1.8 mag(corresponding to more than 2.0 mag d−1 ) on 2022 December 29 was suﬃcient to convince us that the 0.0412 d,
but not its double, is the true period (vsnet-alert 27258).10
These outbursts, however, left us important lessons and we started observations following the detection ofanother outburst at 15.2 mag on 2023 April 29 by RS. FJH obtained time-resolved photometry. The sampling
rate, however, was initially insuﬃcient to detect a period. After increasing the sampling rate on 2023 May 13, thedetected period was conﬁrmed to be the same as in the previous outbursts. The log of observations is summarizedin table 1.

3Long-term behavior

The long-term light curve of ASASSN-22ak using the survey data and visual observations by RS is shown inﬁgure 1. During Gaia observations between 2015 and 2021, the object very slowly faded. This trend was diﬀerent
from V3101 Cyg before the ﬁrst outburst (Tampo et al. 2020). The four outbursts starting from 2022 January
are seen in the right part of the ﬁgure. The quiescent brightness between these outbursts were brighter thanGaia observations before the ﬁrst outburst. The enlarged light curves of these outbursts are given in ﬁgure 2.
Near the termination of the third and fourth outbursts, there were a short (less than 1 d in the third and 2 din the fourth) dip and rebrightening. The presence of such a short dip indicates that the long outbursts wereindeed superoutbursts of a system with a short orbital period, not long outbursts seen in SS Cyg stars. We
should note that the post-outburst observations after the 2022 December outburst (third panel in ﬁgure 2) werebiased brighter since aperture photometry could measure the object only on limited number of frames. The true
magnitudes should be fainter (see the fourth panel in ﬁgure 2, which were observed under more ideal conditions).

2 <https://www.astronomy.ohio-state.edu/asassn/transients.html>.
3 <http://gsaweb.ast.cam.ac.uk/alerts/alert/Gaia22afw/>.
4 <http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-alert/26518>.
5 <http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-alert/26875>.
6 <http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-alert/27223>.
7 <http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-alert/27224>.
8 <http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-alert/27225>.
9 <http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-alert/27243>.
10 <http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-alert/27258>.

3

Table 1: Log of observations of ASASSN-22ak.
Start∗End∗
Mean mag.
59598.2841 59598.3620
2.235
59935.5072 59935.6519
16.04659936.5080 59936.6481
16.04859937.5084 59937.6458
16.13259938.5085 59938.6430
16.15559939.5086 59939.6400
16.26759940.5093 59940.6373
16.34159941.5090 59941.6354
16.45859942.5094 59942.6304
18.28759943.5101 59943.6251
18.60359944.5240 59944.6141
18.96859945.5244 59945.6241
19.01359946.5282 59946.6212
18.93359947.5178 59947.5941
18.67459949.5672 59949.5847
18.39359950.5702 59950.6130
17.83059951.5256 59951.6045
18.29360065.8612 60065.9208
15.366
60066.8591 60066.9203
15.43160067.8564 60067.9177
15.52360069.8509 60069.9176
15.57160070.8480 60070.9215
15.67060071.8453 60071.9189
15.69060072.8425 60072.9221
15.68660073.8382 60073.9208
15.72860074.8355 60074.9227
15.75560075.8342 60075.9208
15.78560076.8298 60076.9222
15.74760077.7851 60077.9234
15.95160078.7822 60078.9245
15.99260079.7794 60079.9239
16.01260080.7767 60080.8968
16.00560081.7739 60081.8964
16.07760082.7711 60082.8968
16.04160083.7690 60083.8965
16.06360085.7670 60085.9041
15.90160086.7615 60086.9034
15.96260087.7579 60087.9040
15.92160088.7551 60088.9048
15.92060089.7523 60089.9052
15.90360090.7495 60090.9050
15.95660092.8064 60092.9059
16.00560093.7412 60093.9065
16.07760094.7384 60094.9061
16.16060095.7357 60095.9062
16.24260096.7328 60096.9070
16.29460097.7301 60097.9073
16.31760098.7273 60098.9071
16.30060099.7245 60099.9079
16.20960100.7217 60100.9077
16.12260101.7190 60101.9078
16.13360102.7162 60102.9082
16.26160103.7134 60103.9086
18.188∗
BJD−2400000.
†
Number of observations.

Error
0.010
0.010
0.008
0.010
0.007
0.009
0.009
0.011
0.065
0.085
0.086
0.083
0.091
0.146
0.224
0.209
0.104
0.029
0.017
0.012
0.020
0.019
0.026
0.011
0.011
0.015
0.025
0.012
0.003
0.003
0.003
0.003
0.037
0.003
0.004
0.004
0.004
0.004
0.002
0.003
0.009
0.003
0.003
0.002
0.003
0.003
0.004
0.004
0.005
0.004
0.004
0.004
0.024

N†
216
53
52
51
50
49
47
47
45
25
16
15
15
10
4
6
13
12
11
11
13
13
13
14
19
20
16
21
178
183
186
155
151
162
84
99
89
202
244
249
232
164
270
274
278
284
289
293
299
303
308
313
315

Observer
MLF
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC

Filter
C
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV

4

Table 1: Log of observations of ASASSN-22ak (continued).
Start∗End∗
Mean mag.
60104.7108 60104.9081
19.27960105.4868 60105.6093
16.65260106.4773 60106.6080
16.69860107.8310 60107.9077
19.80960109.9030 60109.9059
19.83460110.8900 60110.9064
19.84160111.8828 60111.9063
19.681
60112.8835 60112.9057
19.86360113.8873 60113.9123
19.69460114.8995 60114.9265
19.95760117.9123 60117.9315
19.85760118.8955 60118.9272
20.01860119.8928 60119.9246
20.00260120.8901 60120.9267
19.94360121.8871 60121.9266
19.99360122.8845 60122.9269
19.97860124.8790 60124.9262
19.92460125.8780 60125.9261
19.903∗
BJD−2400000.
†
Number of observations.

14

ASN g

Gaia

ATLAS o

Error
0.031
0.009
0.007
0.042
0.332
0.096
0.078
0.081
0.082
0.095
0.088
0.074
0.067
0.069
0.058
0.075
0.071
0.074

N†
171
75
80
66
3
15
19
20
24
18
13
23
22
34
29
31
30
34

ATLAS cObserver
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC
HaC

Filter
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV
CV

visual

16
18
20
57000

57500

58000

58500

59000

Figure 1: Long-term light curve of ASASSN-22ak.

59500

60000

5

14
16
18
20
59580

59600

59620

59640

59660

59680

59760

59780

59800

59820

59840

59860

59880

59900

59920

59940

59960

59980

14
16
18
20

14
16
18
20

14
16
18
20
60060

60080

60100

60120

60140

60160

Figure 2: Light curves the outbursts of ASASSN-22ak. The peaks of the outbursts are not aligned to show theexisting data between the outbursts better. The symbols are the same as in ﬁgure 1. The small dots representnightly averaged unﬁltered CCD magnitudes by FJH. Near the termination of the third and fourth outbursts,
there were a short (0.5–2 d) dip and rebrightening. ASAS-SN did not detect the object 1-d before the initialdetection (upper limit 17.0 mag).

64Superhumps

We analyzed the best observed 2023 outburst. We used locally-weighted polynomial regression (LOWESS: Cleveland 1979) to remove long-term trends. The periods were determined using the phase dispersion minimization(PDM: Stellingwerf 1978) method, whose errors were estimated by the methods of Fernie (1989); Kato et al.
(2010). The result before the dip (2023 June 8, BJD 2460103), after excluding the scattered data on 2023 May
17 (BJD 2460081–2460082) is shown in ﬁgure 3. The period obtained by this analysis is 0.042876(3) d. The
variation of the proﬁles in 2023 is shown in ﬁgure 4. The amplitude of the variations increased on BJD 2460088(2023 June 23), which corresponded to temporary brightening from the fading trend (see ﬁgure 2). Based on theamplitude variation correlated with the overall trend similar to SU UMa stars (Kato et al. 2009) and the gradualshift in the phase of peaks, we identiﬁed these variations to be superhumps, not orbital variations.
An analysis of less observed outburst in 2022 December during the plateau phase is shown in ﬁgure 5. Notethat these 7-night observations recorded only the terminal portion of the outburst and the statistics were notideal. The phase plot assumes a period of 0.042876 d, which is allowed as one of the aliases as seen in the PDM
analysis.

5

Discussion

5.1

Comparison with hydrogen-rich WZ Sge stars

As we have seen, there was no evidence of an outburst in ASASSN-22ak before 2022 (at least for seven years
based on ASAS-SN and Gaia observations). The object suddenly became active and repeated superoutbursts
with cycle lengths of 132–188 d. No very similar object has been known. V3101 Cyg is somewhat analogous inthat it repeated four superoutbursts (up to the time of the writing) following the 2019 large outburst. The caseof V3101 Cyg is diﬀerent in that short rebrightenings were also observed (Tampo et al. 2020). The initial (2019)
outburst of V3101 Cyg showed a relatively rapidly fading phase, which is the viscous decay phase characteristic to
WZ Sge stars (Kato 2015). The initial (2022 January) outburst of ASASSN-22ak had a similar feature, reaching∼2 mag brighter than subsequent outbursts and which apparently faded rapidly. The second and third outburstsof ASASSN-22ak had similar, but less distinct, features. The same feature was almost lacking in the fourth
outburst (ﬁgure 2). These features suggest that the ﬁrst outburst of ASASSN-22ak was a strong WZ Sge-type
one and that the second and third ones were weaker WZ Sge-type ones, although early superhumps (Kato 2015)
were not directly observed during any of these outbursts.
The superhump period of 0.042876 d should be close to the orbital period (see also discussions later). This
period is rather too short for a hydrogen-rich CV. If ASASSN-22ak is a hydrogen-rich CV, the orbital period
should break the record of 0.0462583 d in OV Boo (Littlefair et al. 2007; Patterson et al. 2008; Uthas et al. 2011;
Ohnishi et al. 2019), which is considered to be a population II CV. We consider the possibility of ASASSN-22ak
being a population II CV less likely since the transverse velocity of ASASSN-22ak is 20% of OV Boo (Gaia
Collaboration et al. 2022) (but still with a 28% 1-σ error in the Gaia parallax) and because of the diﬀerencein the light curve (lack of short rebrightenings, long durations of superoutbursts compared to supercycles) fromthe hydrogen-rich V3101 Cyg. ASASSN-22ak would then be more likely a hydrogen-depleted CV. There are
two possibilities. It could be either an EI Psc star (CV with an evolved core in the secondary but still withconsiderable surface hydrogen) or an AM CVn star in which the surface hydrogen of the secondary is almost lost.
We consider these possibilities in more detail.

5.2

Comparison with EI Psc stars in general

EI Psc has an orbital period of 0.0445671(2) d (Thorstensen et al. 2002) very similar to ASASSN-22ak. EI Psc,
however, has a hot, luminous secondary (Thorstensen et al. 2002), whose quiescent color (Gaia GP −RP =+0.88) ismuch redder than in ASASSN-22ak (GP −RP =+0.16). Another EI Psc-type object V418 Ser [superhump period
0.04467(1) d] has GP −RP =+0.52 and this object shows outbursts similar to hydrogen-rich CVs (Kato et al. 2015;
Vogt et al. 2021). The properties of V418 Ser look diﬀerent from those of ASASSN-22ak. CRTS J174033.4+414756(orbital period 0.045048 d) has GP −RP =+0.43 and the outburst behavior (Kato et al. 2014, 2015; Chochol et al.
2015; Imada et al. 2018) appears moderately similar to ASASSN-22ak. CRTS J174033.4+414756 indeed showed abright WZ Sge-type outburst in 2023 February after 5-yr quiescence (vsnet-alert 27373).11 Not suﬃcient time has
11 <http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-alert/27373>.

7

θ
1.00

0.98

0.96

P=0.04288
0.04200.04250.04300.0435

(d)

−0.05

0.00

0.05

−0.5

0.0

0.5

1.0

1.5

Figure 3: Superhumps of ASASSN-22ak in 2023. (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.

8

0.0

(BJD−2400000)

0.0(BJD−2400000)
60087.860096.860078.960088.860097.80.5

0.560079.960089.860098.860080.860090.8

60099.860081.8

1.060092.960082.8

1.0

1.560101.860102.8

1.560085.8

0.560100.860094.8

1.5

0.0

1.0

60093.860083.8

1.5−0.5(BJD−2400000)

60077.9

0.5

1.0

0.0

60095.8

−0.5

0.0

0.5

1.0

1.5

−0.5

0.0

0.5

1.0

1.5

Figure 4: Variation of the superhump proﬁle in 2023. Superhumps of ASASSN-22ak in 2023. The epoch isarbitrarily assumed to be BJD 2460089.800 and the period of 0.042876 d is used.

9

1.05
θ

1.00

0.95

0.90

0.85
0.038

0.040

0.042

0.044

0.046

0.048

(d)

−0.05

0.00

0.05

−0.5

0.0

0.5

1.0

1.5

Figure 5: Superhumps of ASASSN-22ak in 2022 December. (Upper): PDM analysis. The tick is shown at thesuperhump period obtained by the 2023 observations. (Lower): Phase plot.

10
passed since this outburst and it is unknown whether CRTS J174033.4+414756 behaves like ASASSN-22ak. The
known diﬀerences between CRTS J174033.4+414756 and ASASSN-22ak are that the former shows superhumps
with much larger amplitudes, which suggests a higher mass ratio [q=0.077(5) was obtained by (Imada et al.
2018)], and the redder color in quiescence. Although CRTS J174033.4+414756 would be a good candidate for an
already known object having properties similar to ASASSN-22ak, particularly with a bright superoutburst after
5-yr quiescence, the secondary in ASASSN-22ak appears to be fainter and less massive.

5.3

Comparison with CRTS J112253.3−111037

The object most similar to ASASSN-22ak appears to be CRTS J112253.3−111037 (Breedt et al. 2012). Thisobject has an orbital period 0.04530 d and a very small fractional superhump excess ϵ ≡ PSH /Porb − 1, where
PSH and Porb represent superhump and orbital periods, respectively. The secondary in CRTS J112253.3−111037
was undetected in contrast to other EI Psc stars. The Gaia color GP − RP =+0.10 is also very similar to thatof ASASSN-22ak. Although PSH was reported in Kato et al. (2010), this value is vital to this discussion and
we re-analyzed the data in Kato et al. (2010), in which the modern de-trending method was not yet employed.
The resultant period was 0.045409(9) d (ﬁgure 6). This value corresponds to ϵ=0.0024(2). In the treatment byBreedt et al. (2012), old ϵ-q calibrations, which did not consider the pressure eﬀect, were used and they obtained
an exceptionally small q. Using the modern calibration in table 4 of Kato (2022) considering the pressure eﬀect(but calibrated using hydrogen-rich systems), this ϵ corresponds to q=0.043(1) assuming stage B superhumps [forsuperhump stages, see Kato et al. (2009)]. There remains a possibility that the observed superhumps were stage
C ones since observations only recorded the ﬁnal part of the outburst. The periods of stage B superhumps are
generally longer by 0.5% than those of stage C superhumps in hydrogen-rich systems (Kato et al. 2009). If stage B
superhumps were missed and we only observed stage C superhumps, this q value would be an underestimate. By
artiﬁcially increasing the superhump period by 0.5%, the resultant q becomes 0.058(1), which should be regarded
as the upper limit. In actual WZ Sge stars, stage C tends to be missing (Kato et al. 2009; Kato 2015), and we
consider that the ﬁrst value [q=0.043(1)] is expected to be closer to the real one.
CRTS J112253.3−111037 is also similar to ASASSN-22ak in terms of the low frequency of outbursts (Breedt
et al. 2012). There was no information how the 2010 outburst in CRTS J112253.3−111037 started due to an ∼50 dobservational gap in the CRTS data (Drake et al. 2009) and it is unknown whether CRTS J112253.3−111037
showed a sharp peak or a viscous decay phase. No repeated superoutbursts like ASASSN-22ak, however, appear
to have been present since then. It might be interesting to note that ATLAS and ASAS-SN data show that CRTS
J112253.3−111037 showed brightening with a broad peak reaching g=17.8 around 2022 June 6 (BJD 2459737).
The entire event lasted ∼15 d and this may be similar to the enhanced quiescent activity in the AM CVn star
NSV 1440 (Kato and Stubbings 2023), possibly signifying the similarity to AM CVn stars.
The small amplitude of superhumps (0.05 mag) in CRTS J112253.3−111037 is also similar to ASASSN-22ak(0.05 mag), implying a similarly low q in ASASSN-22ak.
Breedt et al. (2012) suggested a possibility that CRTS J112253.3−111037 had already evolved past its
period minimum based on Podsiadlowski et al. (2003) and that its secondary can be semidegenerate. Although
this conclusion was apparently partly based on q smaller than the one obtained in the present paper, we agree thatboth ASASSN-22ak and CRTS J112253.3−111037 are evolving close to AM CVn stars since the properties of theseobjects are very diﬀerent from other EI Psc objects with similar orbital periods (subsection 5.2). ASASSN-22ak
may have already lost hydrogen and it may even be an AM CVn star. If this is the case, ASASSN-22ak breaks
the longest record of orbital periods in AM CVn stars showing a genuine superoutburst [see also the discussion in
Kato and Stubbings (2023); superhump period of 0.0404–0.0415 d in ASASSN-21au = ZTF20acyxwzf (vsnet-alert
25369;12 Isogai et al. 2021; Rivera Sandoval et al. 2022)]. Another AM CVn star with a long orbital period [PNV
J06245297+0208207 in 2023 (Maehara 2023): superhump period 0.035185(8) d (vsnet-alert 2735313 )] showeda superoutburst very similar to ASASSN-21au. This morphology of superoutbursts appears to be common to
AM CVn stars with long orbital periods and the possibility of ASASSN-22ak as being an AM CVn star might
be less likely. We leave this question open since the outburst properties were so unusual in ASASSN-22ak.
In any case, spectroscopy of ASASSN-22ak to determine the hydrogen and helium content and to determine
the orbital period is very much desirable. The addition of ASASSN-22ak seems to strengthen the idea thatcataclysmic variables could be the dominant progenitors of AM CVn binaries (see, e.g., Sarkar et al. 2023; Belloni
and Schreiber 2023).
12 <http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-alert/25369>.
13 <http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-alert/27353>.

11

θ
1.0

0.9

0.8

P=0.04537
0.04450.04500.04550.0460

(d)

−0.05

0.00

0.05

−0.5

0.0

0.5

1.0

1.5

Figure 6: Superhumps in CRTS J112253.3−111037 during the 2010 superoutburst. (Upper): PDM analysis.
(Lower): Phase plot.

12

Table 2: Comparison of objects treated in this paper.
ObjectPeriod (d)∗BP − RP
EI Psc0.0445671+0.88
V418 Ser
0.04467(1)s+0.52
CRTS J174033.4+414756
0.045048+0.43
CRTS J112253.3−111037
0.04530(1)+0.10
ASASSN-22ak
0.042876(3)s+0.16∗
Superscript s refers to superhump period.
†
Value estimated from ϵ assuming stage B.

MG
10.0
10.3(5)11.0(3)11.1(10)12.1(5)q†0.171–
0.077(5)0.043(1)–Superhump amplitude (mag)
0.16
0.06
0.17–0.03
0.05
0.05

The EI Psc-type objects treated in this paper for comparisons with ASASSN-22ak are summarized in table 2.
The mean superhump amplitude for EI Psc was obtained from the data in Uemura et al. (2002). The superhump
amplitudes for V418 Ser and CRTS J174033.4+414756 were from Kato et al. (2015) and Imada et al. (2018),
respectively. Although CRTS J174033.4+414756 showed the initial phase of large superhump amplitudes (Imadaet al. 2018), no such a phase was recorded in ASASSN-22ak. The q values from ϵ assuming stage B were obtainedby the method in Kato (2022).

5.4

Pre-outburst dormancy and repeated superoutbursts

Repeated long superoutbursts with short recurrence times is the unique feature of ASASSN-22ak. In the caseof (hydrogen-rich) V3101 Cyg, some of post-superoutburst rebrightenings may have been caused by the matterin the disk left after the main superoutburst (Tampo et al. 2020). Repeated superoutbursts appear to be moreeasily explained if the mass-transfer rate increased after the initial outburst (Hameury and Lasota 2021). This
increase in the mass transfer may either have been caused by irradiation of the secondary by the initial outburst(Hameury and Lasota 2021), or it could have been that the quiescent viscosity of the disk before the initial
outburst was simply extremely low to accumulate a large amount of mass in the disk and that the mass-transfer
rate and the quiescent viscosity is simply returning to the normal value of this object after the initial outburst.
In the case of ASASSN-22ak, the initial outburst was not as strong as in V3101 Cyg, although the peak
was bright, and the mechanism may be diﬀerent from the case of V3101 Cyg. In ASASSN-22ak, q would besmaller than in V3101 Cyg (as inferred from the smaller amplitude of superhumps and from the analogy with
CRTS J112253.3−111037) and the weaker tidal eﬀect would make it more diﬃcult to maintain superoutbursts incontrast to V3101 Cyg. Although there have been a suggestion that smaller q can lead to premature quenchingof superoutbursts (see e.g., Hellier 2001), there is no established theory when superoutbursts end. Although
this premature quenching of superoutbursts might explain the repeated superoutbursts with relatively shortintervals, the lack of post-superoutburst rebrightenings in ASASSN-22ak might be problematic. It may be thatthe hydrogen depletion in the disk of ASASSN-22ak is not as strong as AM CVn stars and long superoutbursts
are easier to maintain than in almost pure helium disks. A combination of eﬀects of all these circumstances,
unusual for ordinary CVs, should be a challenging target for theorists working with the disk-instability model.
The pre-outburst dormancy might be easier to explain in ASASSN-22ak. In contrast to V3101 Cyg, which is
expected to have a fully convective secondary, ASASSN-22ak has an evolved core and a magnetic dynamo can stillwork (see e.g., Sarkar et al. 2023) and is probably necessary to form the observed AM CVn stars within reasonable
time. With such a dynamo, the instantaneous mass-transfer rate can be diﬀerent from the secular average, as
seen in the spread of absolute magnitudes in CVs above the period gap (Dubus et al. 2018) and the presence of
VY Scl stars. There is also a possibility that the quiescent viscosity of the disk before the initial outburst was
simply very low and the viscosity increased after the outburst as proposed by Osaki et al. (2001); Meyer andMeyer-Hofmeister (2015) for hydrogen-rich WZ Sge stars. This explanation, however, might face a diﬃculty torealize a very quiet, low-viscosity disk when the secondary has a seed magnetic ﬁeld, which may increase thequiescent viscosity of the disk via the magneto-rotational instability (cf. Meyer and Meyer-Hofmeister 1999; but
see also Ishioka et al. 2001). High and low states in polars (AM Her stars: Cropper 1990) may provide additional
insight. EF Eri has a brown-dwarf secondary (Schwope and Christensen 2010, and the references therein) anda strong magnetic activity cycle as in CVs above the period gap is not expected. This object showed (and still13
showing) a long-lasting high state (just like “awakening”) starting from 2022 December (vsnet-alert 27205).14
Since polars do not have an accretion disk, storage of mass in the disk before the active (high) state, as in
WZ Sge stars, is impossible. There could be a reservoir of additional angular momentum other than the disk,
and this might also explain the dormany/waking-up phenomena in dwarf novae.

Acknowledgements
This work was supported by JSPS KAKENHI Grant Number 21K03616. The authors are grateful to the ASASSN, ATLAS and Gaia teams for making their data available to the public. We are also grateful to Naoto
Kojiguchi for helping downloading the ZTF and Gaia data and Yusuke Tampo, 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.
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.
We acknowledge ESA Gaia, DPAC and the Photometric Science Alerts Team (http://gsaweb.ast.cam.ac.uk/
alerts).

List of objects in this paper
OV Boo, AM CVn, SS Cyg, V3101 Cyg, EF Eri, AM Her, EI Psc, VY Scl, V418 Ser, SU UMa, WZ Sge, NSV
1440, ASASSN-21au, ASASSN-22ak, CRTS J112253.3−111037, CRTS J174033.4+414756, Gaia22afw, MASTEROT J030227.28+191754.5, TCP J21040470+4631129, ZTF20acyxwzf

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VSOLJ
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