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\IBVShead{5530}{21 May 2004}

\IBVStitletl{Improved Orbital Ephemeris of GT Ursae Majoris}

\IBVSauth{Miku\v z${}^1$, H.; Dintinjana$^1$, B.;  Zwitter$^1$, T.; Munari$^2$, U.}
\IBVSinsto{${}^1$ University of Ljubljana, Department of Physics, Jadranska 19, SI-1000 Ljubljana, Slovenia}
\IBVSinsto{${}^2$ Osservatorio Astronomico di Padova - INAF, Sede di Asiago, I-36012 Asiago (VI), Italy}

\SIMBADobj{GT UMa}

\IBVStyp{ eclipsing binary }
\IBVSkey{period variation, O-C study}

\IBVSabs{Extensive B and V CCD photometry of GT UMa is reported.}
\IBVSabs{It is used to derive an improved orbital ephemeris and}
\IBVSabs{observe that GT UMa is a detached system of two F2 stars of }
\IBVSabs{equal temperature.}


\begintext
GT UMa ( = HIP 51876, 
$\alpha_{2000} = 10\hr35\mm55\fsec7$, $\delta_{2000} = +63\deg35\arcm32\arcs$) 
is an eclipsing binary with an orbital period 
of 1.16472 days as quoted in the Hipparcos Catalogue Variability Annex. 
The star has no other photometric monitoring reported in the literature. 
So we embarked on a thorough photometric study conducted 
at the \v Crni Vrh observatory, Slovenia in the observing seasons of 
2002 and 2003. The results show that the  orbital period needs 
to be improved. 

We obtained 3713 pairs of Johnson B and V photometric measurements 
of GT UMa during 17 useful nights between 2002 Jan 3 and 2003 Jan 29.
The observations were obtained with a 19-cm, f/4 flat field S-C 
telescope and Wright Instruments Peltier cooled system 
with an EEV CCD02-06-1-206 backside illuminated 
CCD with 574$\times$385 pixels of 22~$\mu$m each.
Exposure time was 30~sec in B and 15~sec in V band.
Measurements have been reduced by the 
\verb|DAOPHOT| (Stetson, 1987) 
package based on 4 comparison stars with colour 
coefficients calculated each night. 
Table 1 quotes names, magnitudes and standard deviations of comparison stars. 
All comparison stars are from the Tycho II catalogue with their 
Johnson magnitudes calculated from the Tycho ones using the 
relations \\$V = V_T - 0.090 (B_T-V_T)$, $B-V = 0.850 (B_T-V_T)$ 
(ESA, 1997). 
We can infer that median error of individual measurements of 
GT UMa is 0.015 in V and 0.011 in B band.
This is in agreement with the dispersion of the light curve of GT UMa 
measured outside of photometric eclipses.

\IBVSfig{17cm}{5530-f1.eps}{Hipparcos and our photometry with orbital phase 
calculated from improved orbital ephemeris (eq.\ 2). }
\IBVSfigKey{5530-f1.eps}{GT UMa}{light curve}


The determination of eclipse minima has been done with the algorithm proposed 
by Kwee \& Van Woerden (1956); the results are presented in Table 2.
We also report some minima from Hipparcos $\mathrm{H}_{\mathrm{p}}$ measurements. Their 
accuracy is lower because of the substantially 
smaller number of points and because individual points are 
separated by at least one rotational period of the satellite.

The original ephemeris based on Hipparcos data 
\begin{equation}
\mathrm{HJD}_{\min} = 2448500.22 + 1.16472 E
\end{equation}
is not consistent with our observations, as timings of our minima 
are displaced for $\sim 0.05$~days. A new ephemeris consistent 
with the whole body of Hipparcos and our measurements is:
\begin{equation}
\mathrm{HJD}_{\min} = 2452278.522(2) + 1.164708(2) E
\end{equation}
Numbers in brackets give errors on the last decimal place. This 
ephemeris is used for the photometry plot (Fig.~1). 
Clearly the improved orbital period satisfactorily joins photometric 
observations by Hipparcos and our new photometry. Note that it 
does not require any secular change of orbital period 
during the time spanned by Hipparcos and our observations.  


\begin{table}
\caption{Comparison stars, average magnitudes and typical 
errors of individual 
measurements. Each star was measured on 3931 (V) or 3934 (B) frames.}
\begin{center}
\begin{tabular}{llllll}
\multicolumn{1}{c}{star}&\multicolumn{1}{c}{B}&\multicolumn{1}{c}{V}&
\multicolumn{1}{c}{B-V}&\multicolumn{1}{c}{$\sigma$(B)}&
\multicolumn{1}{c}{$\sigma$(V)}\\ \hline
TYC 4147 164 1& 10.755& 10.326 &0.429&0.021 &0.036 \\
TYC 4147 600 1& 11.774& 11.031 &0.743&0.027 &0.030 \\
TYC 4147 267 1& 11.764& 10.978 &0.786&0.035 &0.067 \\
TYC 4147 646 1& 11.637& 11.051 &0.586&0.050 &0.080 \\ \hline
\end{tabular}
\end{center}
\end{table}

\begin{table}
\caption{Times of eclipse minima from Hipparcos and our data. \medskip}
\begin{center}
\begin{tabular}{llll}
\multicolumn{1}{c}{type}&\multicolumn{1}{c}{HJD}&
\multicolumn{1}{c}{error}&\multicolumn{1}{c}{filter}\\ \hline
primary&2448474.60  & 0.02 &H$_{\mathrm p}$    \\
primary&2448738.97  & 0.02 &H$_{\mathrm p}$    \\
secondary&2449003.92  & 0.02 &H$_{\mathrm p}$    \\
primary&2452278.52  & 0.005&B and V    \\
primary&2452279.69  & 0.008&B and V    \\
secondary&2452282.60  & 0.005&B and V    \\
primary&2452285.51  & 0.005&B and V    \\
secondary&2452287.255 & 0.006&B and V    \\
primary&2452652.396 & 0.004&B and V    \\
primary&2452654.72  & 0.008&B and V    \\ \hline
\end{tabular}
\end{center}
\end{table}


The light curves are flat-topped with partial eclipses centered on 
0.0 and 0.5 in orbital phase. This indicates that GT UMa is a
detached binary and that photometric data are consistent with 
a circular orbit.
The constancy of the B-V index during eclipses indicates that the two stars 
have an equal temperature. The average value of B-V $=0\fmm36$ corresponds
to the spectral type F2 on the main sequence which agrees with the 
spectral type from the literature.

Hipparcos recognizes GT UMa as a visual binary with another component 
at a separation of 17.570~arcsec, position angle 266\fdeg4, and 
with the same parallax (8.18~mas). The companion star is $\sim 2.6$-mags 
fainter than the eclipsing binary and of a late F or an early G spectral type. 
The separation of the 
components is smaller than the width of the Hipparcos star mapper slits 
(34~arcsec), so light from the visual companion could slightly contaminate 
the measured Tycho magnitudes of the eclipsing binary. 
Halbwachs et al.\ (1997, their Figure~1) showed that this effect is 
smaller than 0.02~mags for the $\mathrm{V}_{\mathrm{T}}$~filter. 
Since the eclipses were sampled at random orientations of the star mapper slit,
the influence on the derived 
relative depth of the eclipses is much smaller, in our case below the 
statistical error of individual $\mathrm{H}_{\mathrm{p}}$ measurements. 
Visual binary is wide enough that it has no influence on the results 
of our CCD photometry.
We note that components of the GT~UMa visual binary have 
a very different size and direction of their proper motion. The implied 
relative velocity projected on the plane of the sky is 
\lower.5ex\hbox{$\; \buildrel > \over \sim \;$} 200$~{\rm km~s}^{-1}$. At a projected separation of 2150~a.u. this 
implies that GT UMa's visual binary is clearly not a bound system, so 
it can have no influence on timing of eclipses and so on orbital ephemeris
of the eclipsing pair. 

We note that a faint ROSAT X-ray source 1RXS~J103554.9$+$633533 (Voges et al.,\
2000) lies very close to the GT UMa visual binary. At the time of 
ROSAT observations (21-24 Oct 1990) the component A (i.e. the eclipsing 
binary) of GT UMa was only 5~arcsec and the component B only 13~arcsec 
from the reported position of the X-ray source. But we note that the positional 
error for $\sim 15$ photons detected by ROSAT is 16~arcsec, so the X-ray 
source may correspond to either of the two components or to 
a completely unrelated source. A relatively small proper motion of 
the A component of GT UMa will keep it within the ROSAT positional errorbox 
for the foreseeable future, so the best chance to resolve the issue is 
a repeated X-ray imaging with superior  
angular resolution capabilities aboard the Chandra satellite. 

 
\references


ESA, 1997, The Hipparcos Catalogue, {\it ESA SP-1200}

Halbwachs, J.-L., Piquard, S., Virelizier, P., Cuypers, J., 
 Lampens, P., Oblak, E. 1997. 
 {\it Proceedings of the ESA Symp. `Hipparcos - Venice '97'}, 
 ESA SP-402, 263

Kwee, K. K., Van Woerden, H., 1956, {\it B.A.N.}, {\bf 12}, 327

Stetson, P. B., 1987, {\it PASP}, {\bf 99}, 191

Voges, W., et al. 2000, {\it VizieR On-line Data Catalog: IX/29}

\endreferences


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