\documentstyle[epsf]{article}


\input ibvs.sty

\def\degr{\hbox{$^\circ$}}   
\def\degm{\hbox{$^{\rm m}$}}
\def\degs{\hbox{$^{\rm s}$}}

\begin{document}
\IBVShead{4204}{6 June 1995}

\title{A suspected K3V variable}\footnotemark
\footnotetext{Based
on observations carried out at the ESO La Silla Observatory}

\begintext

LTPV-B6001 ($\alpha_{2000}=12^{\rm h}26^{\rm m}33^{\rm s},
\delta_{2000}=-62\degr 51'26''$, see finder chart in
Figure 1,  
has been used for more than 7 years as
secondary comparison star for monitoring the X-ray source Wray~977 =
3U\,1223--62 in the framework of the  ``Long-term Photometry of
Variables'' (LTPV) project (Sterken, 1983). Observations were carried
out with the Str\"omgren Automatic Telescope at ESO La Silla, using a
4-channel $uvby$ photometer.  The mean values of 41 measurements in the
so-called ``system~7'' (closely corresponding to the natural system,
see Manfroid et al.,  1992 and Sterken et al., 1993) are $V=  9.687\pm
0.001, b$-$y= 0.892\pm 0.001, m_1=  0.461\pm 0.002, c_1 = 0.278\pm 0.006
$.  No other $uvby$ photometry,  nor Geneva colours are available from
the literature. All data discussed here have been published by Manfroid
et al. (1991, 1994) and Sterken et al. (1995).

\epsfysize=10cm
\centerline{\epsffile{4204-f1.ps}}
\begin{center}
Figure 1. Finder chart for B6001, the position of  Wray~977 and the
error box of 3U\,1223--62 is indicated (source: Vidal 1973)
\end{center}

\vfill \eject

\centerline{2}
\vskip 3mm
\epsfysize=15cm
\centerline{\epsffile{4204-f2.ps}}
\begin{center}
Figure 2. Differential $y$ light curve (in the instrumental
photometric system) with respect to comparison star
HD\,108531 ($V=  8.226, b-y=  0.056, m_1=  0.083, c_1 =0.712 $ )
\end{center}
\vskip 3mm

Though the very small mean errors (due to the large number of
measurements) may suggest that the star is a constant star, an
inspection of the differential $y$ light curve in Fi\-gure 2 shows it is
not: besides some spike-like events, a long-term variation with
amplitude of several hundredths of a magnitude is seen (the
high-frequency noise is observational scatter). The amplitude of
variation increases towards shorter wavelengths, though most of the
increase in amplitude (and scatter) must be attributed to photon noise
since the photon flux drops by more than a factor of 40 from $y$ to $u$.
%(see also van Genderen \& Sterken 1996). 
Another complication that
arises in the bluer bands is the magnitude of the conformity errors
that may arise when combining data from non-congruent photometric
instrumental configurations (note that the star is much redder than the
comparison star HD\,108531). This problem, however, does not affect our
interpretation, since Figure 2 is constructed with
non-transformed data---that is,  data in the instrumental system.
\vfill \eject

\centerline{3}
\vskip 1mm

The accurate mean colour indices allow estimation of the star's
spectral type and luminosity class. We obtain $u-b=2.984$ and the
reddening-free indices $[m_1]=0.755, [c]_1 = 0.100$. This locates  B6001
on the locus of  14 $uvby$ standards and
142 stars within 25 pc  given in the $[c_1], [m_1]$ diagram in Fig.~6 of
Olsen (1984),  and at the extreme end  in the $b-y, u-b$ diagram 
of   these stars in his Fig.~7, quite close to the location of
HD\,95735, a star for which Olsen (1984) gives $\log T_{\rm eff}=3.53$.
B6001, according to its $[m_1]$ and $ [c]_1$ indices, should have
$M_{\rm V}=6.74$ and a spectral type of about K3V or slightly later.

\vskip 1cm

\beginauthors
\authorsrightcol  
C. STERKEN$^1$
University of Brussels (VUB)
Pleinlaan 2
B-1050 Brussels, Belgium
\vskip 2mm
A. van GENDEREN
Leiden Observatory
Postbus 9513
2300RA Leiden, The Netherlands
\vskip 2mm
M. BURGER
Royal Observatory of Belgium
Ringlaan 3
B-1180 Brussels, Belgium
\endauthors
\vskip 2mm
$^1$ Belgian Fund for Scientific Research (NFWO)

\references

Manfroid, J., Sterken, C., Bruch, A. et al., 1991, First
	Catalogue of Stars Measured in the Long-Term Photometry of
	Variables Project (1982--1986), ESO Sc.~Rep.~No.~8 \BIBCODE{1991ESOSR...8....1M}

Manfroid, J., Sterken, C., Cunow, B. et al., 1994, Third
	Catalogue of Stars Measured in the Long-Term Photometry of
	Variables Project (1990--1992), ESO Sc.~Rep.~No.~14 \BIBCODE{1994ESOSR..14....1M}

Olsen, E.H., 1984, {\it A\&AS}, {\bf 57}, 443
% Original: Olsen, E.H., 1984, {\it AAS}, {\bf 57}, 443
% Corrected: Mon Jul 28 09:57:49 MET DST 2003
 
Sterken, C., 1983, {\it The Messenger}, {\bf 33}, 10


Sterken, C., Manfroid, J., Anton, K. et al., 1993, Second
	Catalogue of Stars Measured in the Long-Term Photometry of                     Variables Project (1986--1990), ESO Sc.~Rep.~No.~12 \BIBCODE{1993ESOSR..12....1S}

Sterken, C., Manfroid, J., Beele, D., de Koff, S., Eggenkamp, I.,
G\"ocking, K., Jorissen, A., Kaufer, A., Kuss, C., Schoenmakers, A. P.,
Stil, J. M., van Loon, J., Vink, J., Vrielmann, S., Wa\"elde, E., 1995,
Fourth Catalogue of Stars Measured in the Long-Term Photometry of 
Variables Project (1992-1994), ESO Sc. Rep., No.\,16 \BIBCODE{1995ESOSR..16....1S}

Vidal, N.V., 1973, {\it ApJ}, {\bf 186}, L81


\end{document}

