1No.74

Jul. 2021

ISSN 0917-2211

Variable Star Bulletin
On the superhumps and mass ratio of CzeV404
Taichi Kato1
1Department of Astronomy, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japantkato@kusastro.kyoto-u.ac.jp

Received 2021 Jul. 10

Abstract
CzeV404 is an SU UMa-type dwarf nova in the period gap. Kára et al. (2021) (arXiv:2107.02664) recently
published photometric and spectroscopic observations and obtained a mass ratio q=0.16, which is in severe
disagreement of q ∼0.32 estimated from superhump observations (Bąkowska et al., 2014). I here present what
analysis was wrong or outdated in Bąkowska et al. (2014) and provide a new value of q=0.247(5), consistent
with the known behavior of superhumps and the evolution of cataclysmic variables. CzeV404 does not look
like an unusual dwarf nova as suggested by Kára et al. (2021) and I discuss that the link between SW Sex
and SU UMa systems suggested by Kára et al. (2021) is not supported.

CzeV404 is an SU UMa-type dwarf nova in the period gap (Bąkowska et al., 2014). Kára et al. (2021)
recently published photometric and spectroscopic observations and obtained a mass ratio q=0.16, which is insevere disagreement of q ∼0.32 estimated from superhump observations (Bąkowska et al., 2014).
Here I re-examined the superhump observations in Bąkowska et al. (2014) and solved the discrepancy.
Bąkowska et al. (2014) used all the superhumps timing observations to derive a strongly negative Pdot = Ṗ /P .
Such interpretations were common (e.g. Nogami et al. 2003; Kato et al. 2003; Olech et al. 2003; Rutkowski
et al. 2007) before the establishment of the superhumps stages (A, B and C) in Kato et al. (2009) but are now
outdated. In Kato et al. (2009), it was shown that all previously reported values of large negative Pdot simply
reﬂected the stage transitions of superhumps rather than smooth, monotonous period variations.
CzeV404 (Bąkowska et al., 2014) was a textbook case of stage A-B transition. Their observations started
on the rising branch of the superoutburst (HJD 2456856) and the earliest phase of the superhump development(stage A) was caught up to HJD 2456859. Looking at their light curve (in their ﬁgure 3), it is diﬃcult torecognize superhumps on the ﬁrst night (HJD 856), but growing superhumps were evidently caught on thesecond night (HJD 857). It is likely that their superhump detections for E=0, 1 on HJD 856 were spurious.
Using their superhump maxima for 10≤ E ≤20, I obtained a period of 0.1060(1) d. This value corresponds toϵ∗ ≡ 1 − Porb /PSH , where Porb and PSH represent orbital and superhump periods, respectively, of 0.075(1). Usingthe relation between ϵ∗ and q in Kato and Osaki (2013), this value corresponds to q=0.247(5), which is a very
reasonable value for a cataclysmic variable in the period gap (see e.g. Knigge et al. 2011).
The problems in the analysis by Bąkowska et al. (2014) were: (1) they used the entire phases of superoutbursts, without paying attention to superhump stages and (2) they used an inappropriate formula to estimate q,
which is only applicable to stage B superhumps or novalike stars [but this formula does not consider the eﬀect ofthe pressure eﬀect and should be considered as an experimental formula; see Kato and Osaki (2013) for detaileddiscussions]. The statement in Bąkowska et al. (2014): “The tidal instability (Whitehurst 1988) of the disk starts
to work eﬀectively for binaries with a mass ratio q below 0.25. This assumption was used by Osaki (1989) in theTTI (thermal-tidal instability) model to explain the phenomenon of superoutburstsand superhumps. That is why
such a high value of q in CzeV404 poses a serious problem for thesuperhump mechanism.” is now unsubstantiated.

2I must add, however, there is indeed a case of q=0.31–0.34 object, BO Cet, which showed a superoutburst andsuperhumps (Kato et al., 2021). q=0.25 is not an absolute limit.
Just for completeness, large negative Pdot of MN Dra mentioned in Bąkowska et al. (2014) was a result ofstage A-B transition as clariﬁed by Kato et al. (2016b) and Kato et al. (2016a).
The resultant q=0.247(5) for CzeV404 does not match q=0.16 from eclipse modeling in Kára et al. (2021).
There must have been something wrong with the treatment in Kára et al. (2021) (for example, they assumed
that the disk extends to the tidal truncation radius, which is clearly an overestimate). Eclipses analysis needs tobe redone with better quality data. The statement in Kára et al. (2021): “This is lower than the value estimatedfrom the superhump period applying diﬀerent ϵ ∼ q relationships. However, we note that a similar discrepancy
is also observed in some other eclipsing systems (Kato & Osaki 2013)” is apparently a misunderstanding. Thereis no diﬀerence in the ϵ ∼ q relationship between eclipsing and non-eclipsing systems.
Spectroscopy is outside the scope of this paper and I only make short comments. The spectroscopic resultsby Kára et al. (2021) needs to be treated with caution: (1) It was not clearly described whether they considered
Jacobian in transforming the Doppler tomogram in the velocity space to real coordinates in obtaining their ﬁgure
12. If this was not considered, the resultant map gives an incorrect impression. (2) SW Sex phenomenon isgenerally considered to be seen in systems with high mass-transfer rates (novalike systems above the thermalstability) while the observations by Kára et al. (2021) were made in lower mass-transfer states. We showed thatBO Cet, which had been considered as an SW Sex star (Rodríguez-Gil et al., 2007), had an orbital light curve
not resembling that of an SW Sex star (Kato et al., 2021). The mass of the white dwarf in BO Cet was estimatedto be larger than 1.0M⊙ . If the mass of the white dwarf in CzeV404 is indeed high (∼1.0M⊙ , Kára et al. 2021),
the features resembling those of SW Sex stars in CzeV404 may be a result of a massive white dwarf as in BO Cet.

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Editor Seiichiro Kiyota
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