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%
\def\nsv{NSV\,12597}
\def\h{$^{\rm h}$}
\def\m{$^{\rm m}$}
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\input ibvs.sty

\begin{document}
\IBVShead{4319}{10 April 1996}

\title{NSV 12597 and RX J1957.0+2005}

\begintext

\nsv\, = 59.1928 Sagittae was discovered by Baade (1928) as a variable star
of unknown type in the range 15\fm0--[16\fm7.
The NSV catalogue (Kukarkin \etal\, 1982) lists type E for this object.
The source of this information is unknown, however: we could not find any other
publication on \nsv\, than the one of Baade (1928). So, the statement E
may be a private communication or possibly a misprint.

Unfortunately, no finding chart was given, and the published coordinates 
transform to 

\centerline{R.A. = 19\h57\m01\fss4, Dec. = 20\degs05\amin53\asec.}

\noindent (equinox 2000.0). This is about
7\asec\, north of the eastern star of the unresolved pair denoted with ``v"
in Figure 1. This unresolved pair has a brightness of B$\approx$17\m\,
on the blue sensitive Sonneberg plates, and R$\approx$14\fm5
(B$-$R$\approx$2.2) on the POSS  plate E372 taken in 1951. (Neither B nor R
magnitudes given here are ``international'' values, they rather mean blue and red sensitive photographic magnitudes.)

We checked 310 archival plates of the 400/1600 mm and 400/2000 mm Sonneberg astrographs (limiting magnitude $\approx$18\m) and 
190 plates of the astrograph 170/1400 ($\approx$17\m) for variability of \nsv.
Though difficult to investigate because of its faintness, this star (pair)
seems to be slightly variable between about 16\fm5 and 17\m\, (B band)
during the time span 1938--1995.
Occasionally, we find indications of flat waves ($\approx$40 days) of small 
amplitude ($<$0.3 mag). We do not find eclipses, but given the typical 
exposure times of 20 min. eclipses might easily be undetectable in our data
if they are of short duration and/or of small amplitude.
Though we did not observe any typical flare, the red colour
indicates the possibility that Baade observed a flare-up of \nsv.

We got interested in \nsv\ due to the nearby X-ray source RX J1957.0+2005
discovered during the ROSAT all-sky survey. This X-ray source
with best fit coordinates 

\centerline{R.A. = 19\h57\m02\fss6, Dec. = 20\degs05\amin14\asec,}

\noindent (2$\sigma$ error of $\pm$30\asec) was found at a countrate of 
0.039$\pm$0.010 cts/s. The hardness ratios HR1 = 
(N$_{\rm 52-201}$ -- N$_{\rm 11-41}$)/(N$_{\rm 11-41}$ + N$_{\rm 52-201}$) = 
--0.35$\pm$0.30
(where N$_{\rm a-b}$ denotes the number of counts in ROSAT's position 
sensitive proportional counter between channel a  and channel b)
and HR2 = (N$_{\rm 91-200}$ -- N$_{\rm 50-90}$)/N$_{\rm 50-200}$) = 0.42$\pm$0.64,
though admittedly purely constrained due to the low number of counts,
are consistent with coronal emission from a late-type star. 
Thus, the brightest object inside the X-ray error circle (see Fig. 1),
with B$\approx$13\m\, and located 
at R.A. = 19\h57\m03\fss1, Dec. = 20\degs05\amin10\asec\, is
a viable counterpart candidate nearly independent of its spectral type.
Only spectral types later than about M0 are less probable due to their 
implied low X-ray to  optical flux ratio, and their intrinsic colours
being redder than the B$-$R$\approx$1.5 of this B$\approx$13\m\, star.

A check of this B$\approx$13\m\, star on the same plate material as
described above revealed no hint for any kind of variability.
This makes a possible mis-identification of this B$\approx$13\m\, star
with \nsv\, rather unlikely.

\vfill \eject

\centerline{2}
\vskip 1mm
\epsfysize=8cm
\centerline{\epsffile{4319-f1.ps}}
\normalsize
\begin{center}Figure 1. A finding chart showing the position of NSV 12597 on the
          E band Palomar Observatory Sky Survey plate. ``v" marks the pair 
          of stars, the eastern of which is thought to be \nsv.
          The circle marks
          the 30\asec\, error radius around the X-ray position (cross) of
         RX J1957.0+2005. 
\end{center}
\large

\nsv\ is about 45\asec\, off the position of RX J1957.0+2005.
With the present knowledge we think that \nsv\ is unrelated to 
this X-ray source and that the B$\approx$13\m star is the optical 
counterpart of RX J1957.0+2005. Spectroscopic observations of this 
B$\approx$13\m\, star in the X-ray error box as well as of \nsv\ are needed 
to prove our conclusions and to determine the type of \nsv.

\vspace{0.3cm}
{\it Acknowledgements:} JG is supported by the Deutsche Agentur f\"ur
Raumfahrtangelegenheiten (DARA) GmbH under contract FKZ 50 OR 9201
and GAR by the Deutsches Elektronen-Synchrotron (DESY-PH) under contract
FKZ 05 2S0524. 
The ROSAT project is supported by the German Bundes\-mini\-ste\-rium f\"ur
Bildung, Wissenschaft und Forschung  (BMBW/DARA) and the Max-Planck-Society.
The finding chart (Figure 1) is
based on photographic data of the National Geographic Society -- Palomar
Observatory Sky Survey (NGS-POSS) obtained using the Oschin Telescope on
Palomar Mountain.  The NGS-POSS was funded by a grant from the National
Geographic Society to the California Institute of Technology.  
%The plates were processed into the present compressed digital form with
%their permission.  
The Digitized Sky Survey was produced at the Space
Telescope Science Institute under US Government grant NAG W-2166.

\vskip .7cm

\beginauthors
Gerold A. RICHTER
Th\"uringer Landessternwarte Tautenburg
07778 Tautenburg, Germany
and
Sternwarte Sonneberg
96515 Sonneberg, Germany
\vspace{-1.17cm}
\authorsrightcol
Jochen GREINER
MPI f\"{u}r Extraterrestrische Physik
Giessenbachstr. 1
85740 Garching, Germany
\endauthors
\vskip 5mm
\references
 
Baade W., 1928, {\it Astron. Nachr.}, {\bf 232}, 65 

Kukarkin B.V., \etal, 1982, New Catalogue of Suspected Variable Stars, Nauka,
 Moscow



\end{document}

