Spectroscopic and photometric confirmation of TCP J07151825+4753167 and TCP J15030519+2202041 as hydrogen-rich and helium-rich WZ Sge-type dwarf novae

ATel #17791; Hiromasa Hoke, Keisuke Isogai (Kyoto University), Hiroshi Itoh, Masayuki Moriyama (VSOLJ)
on 10 May 2026; 17:43 UT
Credential Certification: Keisuke Isogai (isogai@kusastro.kyoto-u.ac.jp)

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We report on spectral features seen in the transient TCP J07151825+4753167 on 2026-05-09.534 UT and TCP J15030519+2202041 on 2026-05-09.554 obtained by using a fiber-fed integral field spectrograph (KOOLS-IFU; Matsubayashi et al. 2019, 2025) attached to one of the Nasmyth focuses of the 3.8-m telescope Seimei at Okayama Observatory of Kyoto University. In short, we conclude that TCP J07151825+4753167 and TCP J15030519+2202041 are hydrogen-rich and helium-rich WZ Sge-type dwarfa novae.

TCP J07151825+4753167
According to CBAT, the outburst of TCP J07151825+4753167 was discovered at CV=14.4 mag on 2026-05-09.1599 UT by the NMW-TexasTech survey. There is a likely counterpart in Pan-STARRS and GALEX, with g=21.4, r=21.5, i=21.6, and NUV=21.7. Judging from the outburst amplitude and the brightness of the likely counterpart, this object is expected to be a WZ Sge-type dwarf nova.

We obtained an optical spectrum on 2026-05-09.534 UT by using KOOLS-IFU. The quicklook spectrum shows a blue continuum with the Balmer absorption lines. Also, Hα has a narrow emission core. A blackbody fit to the continuum indicates a temperature of ~13,000 K. From these spectral features, we conclude that this object is a new bright WZ Sge-type dwarf nova.

TCP J15030519+2202041
According to CBAT, the outburst of TCP J15030519+2202041 was discovered at CV=13.9 mag on 2026-05-08.3260 UT by the NMW-TexasTech survey. There is a likely counterpart in Pan-STARRS, with g=21.9, r=22.1, i=21.8; therefore, this object shows an amplitude of ~8 mag. The Asiago Observatory team obtained the spectrum on 2026-05-9.948 UT and reported to CBAT. The spectrum shows He I lines and no Balmer lines. Thus, they proposed to classify this object as a helium-rich dwarf nova.

We also obtained an optical spectrum on 2026-05-09.554 UT by using KOOLS-IFU. The quick look spectrum shows a blue continuum with the He I lines same matching the reported spectrum. In our spectrum, the He I emission line at 5876 Å was detected. Comparing the spectra obtained by the Asiago Observatory and us, the line profile at 5876 Å changed from emission to absorption after 0.494 d. A blackbody fit to the continuum indicates a temperature of ~25,000 K.

In addition, we performed time-series photometry and detected possible early superhumps. The estimated period is 0.0303(1) d, which is shorter than the orbital period of a hydrogen-rich dwarf nova. From these results, this object is indeed a new helium-rich (AM CVn) WZ Sge-type dwarf nova.