Hello Taichi,
Wrong VSX ID of TU Tri
https://www.aavso.org/vsx/index.php?view=detail.top&oid=36621
True:
013915.320 +312419.18 (2000.0) Gaia_DR2_303768056000635776 plx=1.174(0.152) dismod=9.7 pmra=-4.945(0.347) pmdec=-3.627(0.290) G=18.054 BP=17.887 RP=18.200
013915.320 +312419.18 (2000.0) Gaia_EDR3_303768056000635776 plx=1.081(0.121) dismod=9.8 pmra=-4.955(0.158) pmdec=-3.930(0.144) G=18.016 BP=17.929 RP=18.171
013915.352 +312418.98 (2000.0) GALEX_DR5_6373132466490707027 NUV=16.988(0.017) FUV=16.529(0.027)
Your candidate is WD J013915.33+312419.24 = SDSS
J013915.32+312419.2, classified as a DA, but both stars, this one
and SDSS J013915.73+312414.7, are listed as white dwarf candidates
in 2015MNRAS.448.2260G.
They are both blue.
The SDSS magnitudes of SDSS J013915.73+312414.7 are around g= 15.0,
when the star is clearly the fainter one in DSS images and has g=
20.7 in Pan-STARRS1 (your candidate has g= 17.9 in both catalogues).
CMC15 (CMC15 J013915.7+312414) also has an intermediate magnitude
result for the actual variable, at r= 17.2.
The GALEX position for GALEX J013915.2+312419 matches that of the
17.9 star but not is not surprising because this is a hot white
dwarf too, and brighter.
It looks like Gaia had trouble measuring a parallax for this object,
which is surely adding to the confusion, but the Gaia Alerts light
curve is key for the identification:
http://gsaweb.ast.cam.ac.uk/alerts/alert/Gaia20ehj/
A separation of 7" is a piece of cake for Gaia. There is no reason
for a wrong identification in a Gaia detection.
Finally, ZTF (also suitable to solve the two stars) also show a
range 15.7 - 21.2 g, with an outburst peaking around 2458757.
The light curve of your candidate only shows some scatter around g=
17.9 as in the other catalogues.
The CRTS position also agrees with this.
At a 7" separation CSS will show a blended magnitude, so it is not
surprising that the position is that of the brighter star.
Thus, the current identification of TU Tri in VSX is correct and
here we have two white dwarfs only 7" apart.
Cheers,
Sebastian
--
Sebastian Otero
VSX Team
American Association of Variable Star Observers
“The mission of the AAVSO is to enable anyone, anywhere,
to participate in scientific discovery through variable star astronomy”
www.aavso.org