[vsnet-chat 5434] Epsilon Geminorum

["Sebastian Otero" <varsao@fullzero.com.ar>]

Dear all,

For a long time I've been collecting photometry of bright stars in order to
correct mistakes and study probable variability.
Epsilon Geminorum is one of the few stars that show a lot of problems when
trying to get accurate data.
Mean mgnitude and color adopted are V= 2.99, B-V= 1.41.
However, I have noted strong discrepancies in the different photometric
systems that are difficult to explain.
The relations between those systems are not met with this G8Ib star.
Mebsuta (as its proper arabic name is) is classified as a suspected
variable, NSV 3183, but I have found that mesures in the UBV system are
around 2.99 (between 2.97 and 3.01) while in the ubvy system, magnitudes are
between 3.00 and 3.02. In the Geneva system the mean value is 3.033 (At this
color Geneva's V is = Johnson's V).
In the Hipparcos catalogue the V value is even more intriguing, (3.06) but
it is wrong. Given the Ib luminosity class a proper convertion (see my table
at: http://ar.geocities.com/varsao/conversion_entre_hp.htm) gives V= 3.03.
But the spectral type cited in this catalogue is A3mA6-A9 (!!??!!) Where
does it come from? A mistake? A companion? The Epoch photometry doesn't show
variability...

The possibilities are:

1) A secular variable with slow changes in mean magnitude.
2) A star with a peculiar spectrum that makes transformation between systems
more difficult than in most of the cases.

I think the latter is the most probable explanation since the different
values are not from the same system but:
UBV=  V= ~2.99
uvby=   V= ~3.01
Geneva/Hp= V= 3.03

Do some peculiarities exist in epsilon Gem's spectrum that may account for
such differences (= H-alpha absorption enhanced in Lobel and Dupree,
(2000AAS...197.4415L))
I have searched data for thousands of stars in the different systems in my
Photometric Sequences for Bright Variable Stars project and never had such
problems...
Any thought?

Sebastian.


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