1No.79

Aug. 2021

ISSN 0917-2211

Variable Star Bulletin
MP Gem: VY Scl-type star, ﬁnallyTaichi Kato1
1

Department of Astronomy, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japantkato@kusastro.kyoto-u.ac.jp

Received 2021 Aug 18. Revised 2021 Nov 14.

Abstract
MP Gem has long been suspected as a long-period eclipsing binary, which had not been seen in faint statefor 71 years since the discovery in 1944. Its nature has been a mystery. Using Public Data Release of Zwicky
Transient Facility observations, I ﬁnally reached a conclusion that this object is a VY Scl-type cataclysmic
variable by the detection of the deep faint state and the rising phase in 2018.

MP Gem was discovered as a variable star by Hoﬀmeister (1963). Hoﬀmeister (1963) noted that the object
was invisible on a total of nine plates taken on 1944 February 24/25 and 1944 February 25/26. Hoﬀmeister(1963) classiﬁed the variable as “Algol?” (eclipsing binary). No other “eclipse” was recorded on Sonneberg plates
according to Gessner (1973). Böhme (2012) studied further Sonneberg plates between 1981 and 1994 and foundno further event. Böhme (2012) suggested that the object could be an eclipsing binary with a very long period.
Böhme (2014) described further details of the research given in Böhme (2012) and also provided time-resolved
photometry with a CCD camera together with observations by Timo Kantolo. Although short-term variations
were recorded, they could not ﬁnd a period nor obtain a hint for the classiﬁcation of this object.
Böhme (2017) continued observations and found a signiﬁcant drop in brightness in 2016 September, ﬁrst
time in 72 years. The object faded to 19.5 mag on 2016 October 20. Böhme called attention to this object
through the American Association of Variable Star Observers (AAVSO). At that time, Böhme (2017) found thatthe fading was neutral in color and suspected that the object would be an R CrB star. Böhme also reported thisﬁnding to VSNET (Kato et al., 2004) as vsnet-alert 20211 on2016 October 5.1 No further call for attention wasissued through VSNET and I was not aware that this could be an event of a cataclysmic variable (CV).
In 2018, I picked up candidates of CVs from the General Catalogue of Variable Stars (GCVS, Samus’ et al.
2017) using the newly available Gaia parallaxes and colors (currently Gaia Collaboration et al. 2021). MP Gem
was one of them and I obtained a light curve using the All-Sky Automated Survey for Supernovae (ASAS-SN,Shappee et al. 2014, Kochanek et al. 2017). Due to the blending with a nearby star (see Böhme 2017), thedeep fading starting from 2016 September was recorded only as a systematic brightness decrease by ∼1.0 mag.
I was not conﬁdent about the nature of this object at that time although the blue Gaia color (BP =16.05 andRP =15.88) and the faint absolute magnitude (MV ∼ +4.0) were clearly indicative of a CV.
Using Public Data Release 6 of the Zwicky Transient Facility (Masci et al., 2019) observations2 , I foundthat MP Gem was in deep faint state in 2018 around 20.0 mag and slowly returned to the bright state between
2018 August and November (ﬁgure 1). This light curve is characteristic to VY Scl-type novalike CVs, which showfading episodes caused by temporary decreases in the mass-transfer rate (see Hellier 2001, Chapter 12). MP Gem
is now ﬁnally classiﬁed as a VY Scl-type object with rather infrequent fading episodes.
1 http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-alert/20211
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>.

2MP Gem
1618ZTF r
ZTF g20

58200

58400

58600

58800

59000

59200

59400

Figure 1: ZTF light curve of MP Gem. The light curve and blue color are characteristic to a VY Scl-type object.
I noticed that the AAVSO collected a considerable record of the 2016–2018 event in the AAVSO International
Database3 . According to their observations, there was a temporary rise to 17.5 mag in 2017 December to 2018
January. Such temporary brightening during the faint state is frequently seen in VY Scl-type objects (see e.g.
Honeycutt and Kafka 2004).
Historically, eclipsing binaries, R CrB stars and VY Scl-type CVs have often been confused. For an exampleof V504 Cen, see Kato and Stubbings (2003). They can usually be easily distinguished by colors (R CrB stars
also show infrared excesses) and absolute magnitudes when Gaia parallaxes are meaningfully measured.

Acknowledgments
The author is grateful to Naoto Kojiguchi for supplying a wrapper code for obtaining the ZTF data.
This work was supported by JSPS KAKENHI Grant Number 21K03616.
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).

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3 <http://www.aavso.org/data-download>.

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