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AA Vir seems to be a 14th mag. dwarf nova

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Sebastian Otero
Sebastian Otero's picture
AA Vir seems to be a 14th mag. dwarf nova

AA Vir was classified as SR: in the GCVS.
Kiyoshi Kasai called the attention to this star and after checking its light curve it turns out that it has usual faint state around V= 14.8 with very frequent brightenings to V= 13.8.
The raw light curve looks like that of an RRAB star but there is no period in this variability.

It has a J-K= 0.41 so it is not a red variablebut a likely CV.
So here you have another bright CV although it looks like its behaviour shows no surprises excepting the small amplitude.


DDE's picture
AA Vir

Very interesting case! There may be other CVs hidden behind SR: or L: labels in GCVS. If the coordinates of all "old" variables were known to the 1" precision, then the cross-matching with 2MASS could help in identifying other mis-identifications of this kind. For AA Vir it's obvious right from the colors (J=13.42, K=13.00) that this star is not a semiregular red variable. But in other cases when the coordinates are known to 0.1' or worse the ID is not so straightforward (BY Aur was thought to be a Mira for 75 years, but was rediscovered as dwarf nova by MASTER by some luck because its coordinates were off by 0.3', while the robot was checking for the known objects within 3").

The most surprising thing in AA Vir case, however, is its companion star 13.4" away which shows the common proper motion with AA Vir itself!
AA Vir (USNO-B1.0 0907-0225356): pmRA=-34 mas/yr, pmDE=-2 mas/yr
Neibor (USNO-B1.0 0907-0225357): pmRA=-32 mas/yr, pmDE=-8 mas/yr
With the errors of proper motions in USNO-B1.0 being 3-5 mas/yr, these values of PM can be considered identical.

The neighbour is definitely a red dwarf 2MASS 13450481+0042425 (J=15.55+/-0.07, K=14.68+/-0.11) with USNO-B magnitudes B1=19.04 R1=16.54 B2=18.77 R2=17.37 I=16.70. The magnitudes of both stars are also consistent with the CV and RD at rather small distance (~100 pc). This makes AA Vir similar to the dwarf nova in Perseus MASTER OT J042609.34+354144.8 (12.6-16.2m, pmRA=16 pmDE=-42) with a common PM companion (also a red dwarf) at 16.6". If confirmed, these hierarchical triple systems can be used for the precise distance determination (using radio observations similar to those of SS Cyg in outburst) and for calibration of absolute magnitudes of CVs and their red companions. The orbital motion can also be measured by the future astrometric missions. If the stars are at 100 pc distance from us, their orbital period should be in the order of 50,000 years.

DDE's picture
AA Vir

Regarding the variable (AA Vir) itself: it has an ultraviolet counterpart GALEX J134504.3+004254 with quite a large color index: FUV=17.27+/-0.05, NUV=15.75+/-0.01. I think I haven't seen such a large value of (FUV-NUV)=1.5 in cataclysmic variables before, but I don't have a table or a plot with (FUV-NUV) distribution of CVs at hand. Can somebody please comment what does this value tell us about this star?

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