1
No.110Feb. 2023

ISSN 0917-2211

Variable Star Bulletin
On the orbital period of the dwarf nova CW Mon
Taichi Kato1 , Franz-Josef Hambsch2 , 3 , 4
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
3 Bundesdeutsche Arbeitsgemeinschaft für Veränderliche Sterne (BAV), Munsterdamm 90, 12169 Berlin,
Germany
4 Vereniging Voor Sterrenkunde (VVS), Oostmeers 122 C, 8000 Brugge, Belgium
Received 2023 Feb. 20
Abstract
CW Mon is a relatively bright and nearby SS Cyg-type dwarf nova frequently used in detailed analysis of
cataclysmic variables and statistical studies. Using Transiting Exoplanet Survey Satellite (TESS) observations,
we found that the orbital period is diﬀerent from what has been adopted. Using the combined data (TESS,
the Zwicky Transient Facility data and VSNET campaigns), we updated the period to be 0.19346802(4) d.
The previously adopted period of 0.1766 d turned out to be its 2-day alias, probably introduced by a confusion
between the two maxima/minima of the ellipsoidal variations. We conﬁrmed that the object showed grazing
eclipses during the 2016 and 2002 outbursts, and also in quiescence before and after the 2016 outburst. These
eclipses were not necessarily always present and were not remarkable during some past outbursts and in the
TESS data. The presence/absence of eclipses may be related to the disk radius or the brightness of the outerpart of the disk. A 37-min quasi-period oscillation (QPO) signal was reported during the 2002 outburst.
Combined with a recent report of the detection of QPOs around the peaks of long outbursts of a dwarf nova,
we suspect that such QPOs during long outbursts may have been excited when the accretion disk reaches the
maximum radius, the tidal truncation radius as being a possibility.

CW Mon is a relatively bright and nearby (∼330 pc, Gaia Collaboration et al. 2021) SS Cyg-type dwarf
nova. The orbital period of this object was assumed to be 0.1766 d in Kato et al. (2003) based on a preliminary
period of 0.176 d and the possible detection of grazing eclipses by Szkody and Mateo (1986). This period waslisted in Ritter and Kolb (2003) and in AAVSO VSX (Watson et al. 2006),1 and has widely been referenced (e.g.,
Warner 2004; Pretorius and Knigge 2008; Harrison 2016; Hause et al. 2017; Dubus et al. 2018).
One of the authors (TK) found a period diﬀerent from this using Transiting Exoplanet Survey Satellite(TESS) observations (Ricker et al. 2015)2 in quiescence between 2020 December 18 and 2021 January 13. Theseobservations started ∼10 d after fading of the 2020 November–December outburst. No further outburst wasrecorded by 2021 May 4 [The data sources of the long-term behavior were the All-Sky Automated Survey for
Supernovae (ASAS-SN: Shappee et al. 2014), the Asteroid Terrestrial-impact Last Alert System (ATLAS: Tonry
et al. 2018) forced photometry (Shingles et al. 2021), the Zwicky Transient Facility (ZTF: Masci et al. 2019)3
and observations reported to VSOLJ and VSNET (Kato et al. 2004)]. The presence of the orbital variation
was already visible to the eyes in the TESS light curve. After removing the global trend by locally-weighted
1 <https://www.aavso.org/vsx/index.php?view=detail.top&oid=18951>.
2 <https://tess.mit.edu/observations/>. The full light-curve is available at the Mikulski Archive for Space Telescope (MAST,<http://archive.stsci.edu/>).
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>.

2

θ
1.0

0.8

0.6

P=0.19347
0.4
0.185

0.190

0.195

0.200

(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 1: Period analysis of the TESS data in quiescence. (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. The zero phase (likely inferior conjunction of the secondary) is BJD 2459214.651.

3polynomial regression (LOWESS: Cleveland 1979), a phase dispersion minimization (PDM, Stellingwerf 1978)
analysis yielded an orbital period (ﬁgure 1) having a period of 0.193467(4) d with its error determined by themethods of Fernie (1989) and Kato et al. (2010). The zero orbital phase (BJD 2459214.651) was chosen as
the shallower minimum of the ellipsoidal variations, which likely corresponds to the inferior conjunction of thesecondary reﬂecting the eﬀect of gravitational darkening. The amplitudes of the ellipsoidal variations were small
in the TESS data due to contamination of nearby stars. This period could not be detected in the ATLAS forced
photometry data (2145 measurements). A PDM analysis of the ZTF data (quiescent parts only, after removing
the trends by LOWESS) detected a period of 0.193468(1) d (ﬁgure 2) (due to the large scatter in the individual
data, we only show a phase-averaged light curve of the combined g+r data). This light curve appears to show
an orbital hump.
The same period was detected from the data obtained during and around the 2016 October outburst (observer: FJH). The phase-averaged light curve (ﬁgure 3) in quiescence before and after the outburst indicates thepresence of an eclipse and an orbital hump. This ﬁnding apparently reinforces the presence of grazing eclipses
reported during the 2002 outburst (Kato et al. 2003). Using all the data (TESS, ZTF and the 2016 VSNET
campaign) between 2002 and 2022, we obtained a reﬁned ephemeris of:Min(BJD) = 2459214.651 + 0.19346802(4)E.

(1)

Using this ephemeris, the corrected ﬁgure corresponding to ﬁgure 4 in Kato et al. (2003) is shown in ﬁgure
4. Although the disappearance of an eclipse on the second night during the 2002 outburst was likely due to theusage of an incorrect orbital period, not the change in the disk radius suggested in Kato et al. (2003), this could
not be directly conﬁrmed due to the absence of the data on the second night around the expected eclipse. The
two periods of 0.1766 d and 0.19346802 d are in the relation of 2-day alias, which was probably introduced bya confusion between the two maxima/minima of the ellipsoidal variations by Szkody and Mateo (1986). The
same ﬁgure for the 2016 outburst is shown in ﬁgure 5. Eclipses were also present. The available data seem to
conﬁrm that CW Mon is a grazing eclipser both in outburst and in quiescence, at least in quiescence before andafter the 2016 outburst. The eclipse signature in the TESS data was less apparent than in 2016. Although there
were some observations during other outbursts in 2005 (Hiroyuki Maehara), 2008 (Seiichiro Kiyota) and 2010 (S.
Kiyota), the results were not as clear as in 2016. An eclipse-like signal was possibly present at the expected phaseon two nights, but absent in one night in 2005. The data were insuﬃcient in 2008 and the eclipse signature wasapparently absent in 2010 in one night covering this phase. These observations were relatively short and did notcover much of outbursts (and they did not record quiescence before and after the outburst) as in 2016, and we feel
it diﬃcult to draw a ﬁrm conclusion. It is possible that the eclipse signature is missing during some outbursts.
The 2002 and 2016 outbursts had a “shoulder”, or an embedded precursor (Cannizzo 2012), sometimes seen inlong outbursts of dwarf novae above the period gap and the disk was probably large during such outbursts. In
other outbursts, either the size of the disk may have not been suﬃcient or the outer part of the disk may havenot been bright enough to show detectable eclipses. This interpretation may be tested by observations of future
observations of diﬀerent types of outbursts.
Regarding the possible signal of an intermediate polar (Kato et al. 2003), Pretorius and Knigge (2008) did notdetect a coherent pulsation nor a spectroscopic signature of a magnetized white dwarf. Warner (2004) classiﬁed
the reported 37-min signal as quasi-periodic oscillations (QPOs). There has been increasing evidence that shortterm variations are excited around a shoulder (or an embedded precursor). For example, quite recently, Sun et al.
(2023) reported enhancement of QPO signals near the long outburst top light curves in HS 2325+8205 = NSV
14581 using the TESS data. These outbursts had a shoulder. Similar enhancement of short-term variations (anorbital or a superhump signal) during a shoulder of a long outburst was also reported in V363 Lyr (Kato 2021)
using the Kepler data. The QPOs observed in CW Mon might have been related to these phenomena, and may
be related to excitation of short-term variations when the accretion disk reaches the maximum radius, the tidal
truncation radius as being a possibility.

Acknowledgements
This work was supported by JSPS KAKENHI Grant Number 21K03616.
The author is grateful to the TESS, ATLAS, ZTF and ASAS-SN teams for making their data available to the
public. I am grateful to Naoto Kojiguchi for helping downloading the TESS data. I am also grateful to VSOLJ
and VSNET observers who reported the data of CW Mon, particularly to Hiroyuki Maehara and Seiichiro Kiyota
for providing time-resolved photometric data during past outbursts.

4

θ
1.00

0.95

0.90

P=0.19347
0.19250.19300.19350.1940

(d)

−0.2

0.0

0.2

−0.5

0.0

0.5

1.0

1.5

Figure 2: Period analysis of the ZTF data in quiescence. (Upper): PDM analysis. (Lower): Phase plot. The
zero phase is the same as in ﬁgure 1.

5

θ
1.0

0.9

0.8

P=0.19347
0.7
0.185

0.190

0.195

0.200

(d)

−0.2

0.0

0.2

−0.5

0.0

0.5

1.0

1.5

Figure 3: Period analysis of the data before and after the 2016 October outburst (BJD 2457669–2457671 and
BJD 2457683–2457711). (Upper): PDM analysis. The vertical tick is given at the orbital period derived from theentire data discussed in this paper. (Lower): Phase plot. The zero phase is the same as in ﬁgure 1. An eclipse
and an orbital hump are apparently seen.

60.0(BJD−2400000)

52582.052583.0

0.552584.052585.0

1.052588.052589.0

−0.5

0.0

0.5

1.0

1.5

Figure 4: Nightly orbital proﬁles during the 2002 outburst using the correct orbital period. A discontinuity inthe light curve of BJD 2452588.0 was an artiﬁcial one caused by two diﬀerent observers giving diﬀerent trends.

7

0.0(BJD−2400000)

57673.057674.0

0.557675.057676.0

1.057677.057678.0−0.5

Figure 5:nights.

0.0

0.5

1.0

1.5

Nightly orbital proﬁles during the 2016 outburst. Eclipses were visible at least on the third to ﬁfth

8This work has made use of data from the Asteroid Terrestrial-impact Last Alert System (ATLAS) project.
The Asteroid Terrestrial-impact Last Alert System (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 the survey 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 Institute of 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
SS Cyg, V363 Lyr, CW Mon, NSV 14581, HS 2325+8205

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