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\IBVShead{5211}{13 December 2001}

\IBVStitletl{Photometric Variability of the Stars}{ USNO 0900-17903132, HD191674 and HD 191616}

\IBVSauth{Robb$^1$, R. M.; Lindner, J. V.; Stern, L. A.; Lindner, T. H.; Balam$^{1,2}$, D. D.; Clem,~J.~L.}

\IBVSinst{Guest User, Canadian Astronomy Data Centre, which is operated by
the Herzberg Institute of Astrophysics, National Research Council of
Canada}

\IBVSinst{Guest Observer, Dominion Astrophysical Observatory, which is
operated by the Herzberg Institute of Astrophysics, National Research
Council of Canada}

\IBVSinsto{Climenhaga Observatory, Dept.\ of Physics and Astronomy, 
University of Victoria, Victoria, BC, Canada, V8W 3P6, 
Internet: robb@uvic.ca}

\IBVSabs{HD191674 has a 0.08 magnitude brightness variation with a period}
\IBVSabs{of 5.2 days. HD191616 has a 0.11 magnitude brightness variation}
\IBVSabs{with a period of about 18 days.}

\SIMBADobj{HD191616}
\SIMBADobj{HD191674}

\SIMBADobj{USNO-A2.0 0900-17903132}

\IBVStyp{ BY }

\IBVSkey{stars: activity}
\IBVSkey{stars: RXJ201043+045449}
\IBVSkey{stars: HD191616}

\begintext

As part of a continuing search for photometric variations in stars which
are known X-ray sources, we observed HD~191616 (=GSC 0503-0700). This star,
also known as RXJ201043+045449, was discovered to be a source of X-rays by
the ROSAT satellite (Voges et al. 1999).  The Tycho catalog (ESA 1997)
reported $V_T$=9\fmm497 and $B_T$=10\fmm724 for HD~191616 and $V_T$=9\fmm000 and $B_T$=10\fmm238
for one of the field stars, HD~191674 (=GSC~0503-1409), both consistent
with a spectral type of approximately K0.  The reported parallaxes favor
dwarf luminosity class but do not exclude giant status.

\IBVSfig{6cm}{5211-f1.eps}{Finder chart labeled with the GSC
identification numbers from region 0503.}

The field of stars observed with the automated 0.5m telescope of the
Climenhaga Observatory at the University of Victoria is shown in Figure 1.
The data were reduced in a fashion similar to that described in Robb and
Greimel (1999). Table 1 lists the stars' identification numbers,
coordinates (J2000) and magnitudes from the Hubble Space Telescope Guide
Star Catalog (GSC) (Jenkner et al., 1990).  Observations were made using 
filters closely matching the Cousins R and I bands (Cousins 1981). The
thirty-two Julian Dates of observation ($-$2450000) are 2121, 2122, 
2129-2138, 2150, 2151, 2157-2166, 2168, 2169, 2175, 2176, 2179, 2180,
2182 and 2184. 

Our differential $\Delta R_c$ magnitudes are calculated in the sense of
the star minus GSC~0503-1521.  Brightness variations during a night were
measured by the standard deviation of the differential magnitudes and are
listed for the most photometric night in the last column as ``Std Dev
Within''. For each star the mean of the nightly means is shown as $\Delta
R_c$ in Table 1.  The standard deviation of the nightly means is a measure
of the night to night variations and is called ``Std Dev Between" in
Table 1.

\begin{table}
\caption{Stars observed in the field of HD~191616=GSC 0503-0700
and HD~191674=GSC~ 0503-1409}
\begin{center}
\begin{tabular}{ccccccccc}
\hline
Star & R.A. & Dec. & GSC & $\Delta R_c$ & Std Dev & Std Dev& $(R-I)$ \\
Id & J2000&J2000 & Mag.& Mag.      & Between & Within & \\

\hline

HD~191616&$20\hr10\mm44\sec$&$4\deg$54\arcm50\arcs&8.8&0.490&.022&.004&0.4\\
 GSC~0503-1521&$20\hr10\mm51\sec$&$4\deg$56\arcm57\arcs&8.6&-&-&-&0.4\\
HD~191674&$20\hr11\mm03\sec$&$4\deg$55\arcm10\arcs&9.3&$-$.222&.025&.003&0.4\\
GSC~0503-1318&$20\hr10\mm38\sec$&$4\deg$54\arcm19\arcs&10.0&1.447&.003&.003&0.4\\
GSC~0503-388&$20\hr10\mm35\sec$&$4\deg$54\arcm55\arcs&11.9&3.428&.004&.007&0.2\\
GSC~0503-328&$20\hr10\mm43\sec$&$4\deg$56\arcm09\arcs&11.7&3.060&.004&.009&0.4\\
USNO~900-17903132&$20\hr10\mm43\sec$&$4\deg$55\arcm51\arcs&13.9&4.787&.157&.036&2.3\\
GSC~0503-802&$20\hr10\mm54\sec$&$4\deg$56\arcm45\arcs&12.4&3.474&.002&.007&0.3\\
GSC~0503-2279&$20\hr10\mm47\sec$&$4\deg$58\arcm44\arcs&13.1&4.579&.011&.019&0.2\\

\hline
\end{tabular}
\end{center}
\end{table}
 
The ``Std Dev Between'' for stars [GSC 0503-1318 - GSC 0503-1521] is
0.003 magnitudes. This excellent photometry shows that night to night
variations in either of these stars must be less than a few
millimagnitudes.  We observed no significant variations in these stars in
plots of the individual nights' data and a ``Std Dev Within" one night
of 0.003 sets an upper limit on variations of an hourly timescale.

\IBVSfig{4cm}{5211-f2.eps}{$\Delta R_c$ filtered light curve of USNO 
0900-17903132 for summer 2001}

In Figure 2 we have plotted the nightly mean and standard deviation of the
$\Delta R_c$ brightness of the star USNO 0900-17903132.  Obviously it is a
variable star with a timescale of at least a hundred days and from our
$(R-I)_c$ we estimate a spectral type of late M.  We expect this star to
be a long period variable and urge observers to monitor its brightness and
discover its amplitude and periodicity, if any.

\IBVSfig{4cm}{5211-f3.eps}{ Periodogram for HD~191674 in 2001}

The star HD~191674 had obvious night to night variations. The
root-mean-square (RMS) deviation of a data point from a sine curve as a
function of period is shown in Figure 3.  From this periodogram we find
the ephemeris to be:

\vspace {3mm}
\centerline{HJD of Maximum Brightness=
2452118\fday3(9)+5\fday21(15)$\times$E.} 
\vspace {3mm}

\noindent where the uncertainties in the final digit are given in brackets
and the RMS error of the fit is less than 0.01 magnitudes. The 3685
differential $\Delta R_c$ magnitudes phased at this period are
plotted in Figure 4 with different symbols for each cycle.

\IBVSfig{6cm}{5211-f4.eps}{$R_c$ filtered light curve of HD~191674}

There are significant differences from cycle to cycle but doubling the
period did not improve the fit. The $(R-I)_c$ is consistent with the Tycho
$B_T$ and $V_T$ measurements indicating that the star is approximately K0
spectral type.  Therefore we postulate that HD~191674 is a late type
spotted dwarf rotating with a 5\fday21 period.

\IBVSfig{4cm}{5211-f5.eps}{$\Delta R_c$ filtered light curve of
HD~191616 for 2001 with model curve overplotted}

Our observations of HD~191616 are plotted in Figure 5. with the error bars
set to the standard deviation of the points during each night.  The error
of the nightly mean then would be about ten times smaller. Obviously
HD~191616 is also a variable star, so Period98 (Sperl 1998) was used to
find the period, amplitude and shape of the variations. The best model
light curve found is of the form:

\centerline{ 
$\Delta$R=0.4611+0.0554*sin(2$\pi$(JD$-$2112.0)/18.4638)+0.0198*sin(2$\pi$(JD$-$2113.572)/9.2319)}

Note that the second period was set to exactly half the dominant period,
to modify the shape of the curve to best match the asymmetry in the
distribution of the spots on the surface of the star.  Spectroscopic
observations made with the DAO 1.8m telescope were used to determine the
spectral class to be K3, consistent with the $(R-I)_c$ and Tycho
measurements.  The spectrum displayed in figure 5 clearly shows the
H$\alpha$ emission line.

\IBVSfig{4cm}{5211-f6.eps}{Spectrum of HD~191616 showing the H$\alpha$
emission line at $6563\AA$.}

HD~191616 is an X-ray source, 
a K3 spectral type star with H$\alpha$ emission,
photometric variations with an $18\fday5\pm.4$ period 
and cycle to cycle differences
typical of BY Dra stars.
Photometric observations should be continued to monitor for flares, changes
in the spot distribution and to determine the period more precisely. 


\vskip 1cm
\references

Cousins, A.W., 1981, SAAO Circ., {\bf 6}, 4

ESA, 1997, The Hipparcos and Tycho Catalogues, ESA SP-1200

Jenkner, H., Lasker, B., Sturch, C., McLean, B, Shara, M., Russell, J., 
1990, {\it AJ}, {\bf 99}, 2082

Robb, R.M. and Greimel, R., 1999, {\it ASP Conf. Ser.}, {\bf 189}, 198

Sperl, M., 1998, Master's Thesis, Institut f\"ur Astronomie, Universit\"at
Wien

Voges, W., et al., 1999, {\it A\&A}, {\bf 349}, 389

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