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\IBVShead{5310}{3 September 2002}

\IBVStitletl{Light Curve Variability in XZ Canis Minoris}

\IBVSauth{Terrell, D.$^1$; Henden, A.A.$^2$}

\IBVSinst{Dept. of Space Studies, Southwest Research Institute,
1050 Walnut St., Suite 400, Boulder, \\ \indent  CO 80302, USA,
e-mail: terrell@boulder.swri.edu}

\IBVSinst{Universities Space Research Association/U.S. Naval
Observatory Flagstaff Station, P.O. Box 1149, Flagstaff,  \\ \indent AZ 86002-1149,
USA, e-mail: aah@nofs.navy.mil}

\SIMBADobj{XZ CMi}
\IBVStyp{ EB }
\IBVSkey{photometry}
\IBVSabs{We report on variability of the light curve of the short-period}
\IBVSabs{Algol XZ CMi as well as evidence for a recent period change.}
\IBVSabs{We also present the first $U$, $R_{c}$ and $I_{c}$ light curves of the system.}

\begintext

XZ CMi is a short-period Algol that has previously been observed by Wilson (1966) and
Terrell, Gunn, and Kaiser (1994; hereafter TGK) in the $B$ and $V$ passbands. In hopes of determining 
better system parameters and exploring the possibility of third light in the system 
as claimed by Rafert (1990), we decided to obtain $UBVR_{c}I_{c}$ photometry during the
2001-2002 observing season. XZ CMi was observed in the $V$ passband from December, 
2001 with the 24" telescope and ST-8 CCD at Sommers-Bausch Observatory (SBO) on the University of Colorado 
campus. The observation continued
with the USNOFS 1.0m telescope and SITe CCD from February, 2002 and 
complete $UBVR_{c}I_{c}$ light curves were obtained.

Rafert (1990) used the Wilson-Devinney program (WD; Wilson and Devinney, 1971; Wilson, 1979) to
fit the Wilson (1966) light curves. He found that third light in the amounts of 17\% in $V$
and 11\% in $B$ yielded better fits to the data than zero third light. On the other hand, Terrell 
and Wilson (1990) and TGK found acceptable fits without third light. Clearly the third light
hypothesis was not strongly tested with data available in these previous studies. Based on our
new observations, we can now state 
that third light was definitely present in these previous datasets. Figure 1 shows part of an $I_c$ band
image taken by Henden and clearly shows a visual companion 2.4" east and 0.5" south of XZ CMi.

Henden performed PSF-fitting to images taken on a night of relatively good seeing to measure
the positions and colors of XZ CMi and the companion star. The data are listed in Table 1. Astrometry 
is based on the USNO-A 2.0 catalog and has less than 100 mas internal errors. The photometry has errors 
of 0.01 magnitudes or less. More complete photometric information about all stars within 5 arcmin 
of the variable can be found in file 5310-t3.txt at the IBVS web site. \IBVSedata{5310-t3.txt}
%IBVSdataKey{5310-t3.txt}{XZ CMi}{sequence}

\begin{center}
{Table 1. Standard magnitudes and color indices} \vskip2mm 
\begin{tabular}{cccccccc}
\hline
Star & RA (J2000) & Dec (J2000) & $V$ & $B-V$ & $U-B$ & $V-R_{c}$ & $R_{c}-I_{c}$\\ 
\hline
XZ CMi & 07:54:07.09 & +03:39:20.6  & 10.122 & 0.371 & $-$0.043 & 0.197 & 0.259 \\ 
Companion & 07:54:07.24 & +03:39:20.1  &  12.097 & 0.663 &  & 0.416 & 0.404 \\
\hline
\end{tabular}
\end{center}
  
\IBVSfig{10cm}{5310-f1.eps}{I-band image showing the fainter companion of XZ CMi.}
%IBVSfigKey{5310-f1.eps}{XZ CMi}{finding chart}

As previously noted, our two datasets were obtained about two months apart. During that time
the $V$ light curve of XZ CMi changed noticeably. The SBO data show a mild asymmetry of about
0.01 magnitudes in the two maxima, with the maximum following secondary eclipse (i.e. phase 0.75)
being the dimmer. By the time of the USNO observations, the asymmetry had grown to about 0.05 magnitudes. Previous 
datasets (Wilson, 1966 and TGK) were of lower precision but showed indications of slightly asymmetric
maxima. Figure 2 shows our two $V$ datasets. It is clear that the asymmetry in the light curves is 
affecting the maximum following secondary eclipse since the minima and the maximum following primary
eclipse of the two datasets match up well. The most likely explanation for the asymmetries is the 
presence of large spots on the secondary star as is common in rapidly rotating stars with 
convective envelopes. 

\IBVSfig{10cm}{5310-f2.eps}{$V$ light curves of XZ CMi. The arrows and vertical lines
indicate the range of data used in the Wilson-Devinney solution.}
%IBVSfigKey{5310-f2.eps}{XZ CMi}{light curve ; light curve: model}

We performed a solution to our $V$ observations using an unreleased version of the WD program
that uses Kurucz atmosphere models to model the radiation of the stars. We used only data unaffected
by the asymmetries, specifically data from phases 0.95 to 0.55. Because the most recently published
ephemeris for the system (TGK) was obviously not valid for the current epoch, we adjusted the period
and heliocentric Julian date of the primary minimum in our least squares fits. The fits were done in
WD mode 5 which constrains the secondary star to fill its Roche lobe. Both of our datasets were
obtained using aperture photometry and included the light of the companion star, so we fixed the third 
light to be 15\% of the total system light at phase 0.25, based on the results of our PSF fits. 

There has been some disagreement about the spectral type of XZ CMi. The GCVS lists it as F0 but Wilson
(1966) argued that his photometry supported an A5 classification. Our photometry during secondary 
eclipse indicates a spectral type of about F2. Accordingly, we set $T_1$ to 7000 K in our solution. 
Unadjusted parameters such as the gravity darkening exponent and bolometric albedo were set to their
theoretically expected values assuming that the envelope of the primary star was radiative and the secondary's
was convective. The logarithmic limb darkening law was used with coefficients interpolated from the 
Van Hamme (1993) tables.

\begin{center}
{Table 2. Adjusted Parameters for Light Curve Solution} \vskip2mm 
\begin{tabular}{ccc}
\hline
Parameter & Value & Std. Error\\ 
\hline
$i$ & 78\fdeg8 & 0\fdeg2 \\ 
$T_2$ & 4910 K & 20 K \\ 
$q$ & 0.68 & 0.02 \\ 
$\Omega_1$ & 3.48 & 0.04 \\ 
$L_1/(L_1+L_2)_V$ & 0.85 & 0.05 \\
$HJD_0$ & 2451957.6748 & 0.0011 \\
$P$ & 0\fday578852 & 0\fday000002 \\
\hline
\end{tabular}
\end{center}

Table 2 shows the results of our WD solution. The errors are the standard errors (1$\sigma$) from the least squares solution. The main differences between our solution and that of
TGK are the higher mass ratio (0.68 versus 0.42) and a lower luminosity ratio (0.85 versus 0.92). A 
slight difference in the luminosity ratio is not surprising since the version of WD used by TGK used 
monochromatic effective wavelengths whereas the newer version that we employed uses actual filter bandpasses, and 
we included third light while TGK did not. The difference in the mass ratios is more troubling, an 
indication that perhaps the photometric mass ratio is not well-determined in this partially eclipsing 
system. We hope to obtain radial velocities of the system, as well as further photometry,
during the next observing season.

The period derived from our light curve solution, 0.578852 days, is significantly larger than that found 
by TGK, 0.578809 days, and by Wilson (1966), 0.578811 days. Period changes are not unexpected since 
XZ CMi is semidetached and shows signs of large-scale magnetic activity. We encourage observers to 
measure frequent times of minimum to help characterize the nature of the period changes during this 
period of apparently heightened activity.

The SBO data are available from the IBVS web site as 5310-t4.txt and the USNO data are available as
file 5310-t5.txt. \IBVSedata{5310-t4.txt} \IBVSedata{5310-t5.txt}
%IBVSdataKey{5310-t4.txt}{XZ CMi}{photometry}
%IBVSdataKey{5310-t5.txt}{XZ CMi}{photometry}

DT wishes to acknowledge the continued support of Keith Gleason at SBO for a generous
allocation of time on the SBO 24" for this and other binary star projects.

\references

Rafert, J.B., 1990, {\it AJ}, {\bf 100}, 1253

Terrell, D. and Wilson, R.E., 1990, {\it PASP}, {\bf 102}, 646

Terrell, D., Gunn, J.B. and Kaiser, D.H. (TGK), 1994, {\it PASP}, {\bf 106}, 149

Van Hamme, W., 1993, {\it AJ}, {\bf 106}, 2096

Wilson, R.E., 1966, {\it AJ}, {\bf 71}, 32

Wilson, R.E., 1979, {\it ApJ}, {\bf 234}, 1054

Wilson, R.E. and Devinney, E.J. (WD), 1971, {\it ApJ}, {\bf 166}, 605

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