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\IBVShead{5145}{25 July 2001}

\IBVStitletl{Photometric Observations of the Extreme Mass Ratio,}{ High Contact Dwarf Binary V902 Sagittarii}

\IBVSauth{Samec, Ronald G.$^{1,2}$; Corbin, Scott F.$^1$}

\IBVSinst{Astronomy program, Dept.\ of Physics, Bob Jones University,
Greenville, SC 29614 USA,\\ \indent email: rsamec@bju.edu}

\IBVSinst{Visiting Astronomer, Cerro Tololo Inter-American Observatory,
National Optical Astronomy Observatories, which are operated by the
Association of Universities for Research in Astronomy, Inc.\ under
contract with the National Science Foundation}

\SIMBADobj{V902 Sgr}

\begintext

Ferwerda (1941, 1943) discovered V902 Sagittarii [V105, $a(2000) = 
19\hr 25\mm 14\fsec155$, $d(2000) = -29\deg09\arcm 08\farcs740$] in his
study of faint variables near $\tau$ Sgr. An ephemeris ($P = 
0\fday2939444$), a finding chart, comparison stars and 28 times of
minimum light are given in his paper. His photographic light curve
gives evidence that V902 Sagittarii is a rare, high fill-out, small
mass ratio, over contact binary. Ben (1943) suggested that V902 Sgr
might be an RR Lyr type. Since this object appeared to be an
interesting variable in need of further study, it was included as a
target for our 1993 observing run at Cerro Tololo InterAmerican
Observatory in Chile. Our present observations were taken with the
1.0-m Yale Reflector and a Ga-As PMT and standard filters on July
22-23, 1993, by RGS\@. Around 300 observations were taken in each pass
band. Our $I$ curves have higher scatter than the others. The
comparison star (GSC 6888 991, RA$(2000) = 19\hr24\mm33\fsec630$,
DEC$(2000) = -29\deg12\arcm10\farcs30$) and the check star (GSC 6888
1052, RA$(2000) = 19\hr24\mm36\fsec541$, DEC$(2000) = 
-29\deg12\arcm22\farcs06$) are shown in Figure~1 as COMP and CHK, with
the variable, VAR\@. Kwee (1962), gave a photographic magnitude range
of 14.4 to 14.78 for V902 Sgr.

\begin{table}[!h]
\caption{Time of Minimum Light, V902 Sagittarii} 
\begin{center}
\begin{tabular}{lcrr}
\hline
JD Hel. & \lower6pt\hbox{Min} & \lower6pt\hbox{Cycles} & \lower6pt\hbox{$O-C$} \\[-4pt]
2440000\,+ \\
\hline
9190.6293(2) & I & $-3.5$ & $-0.0018$ \\
9190.7797(15) & II & $-3$\phantom{.0} & 0.0016 \\
9191.6589(3) & I & 0\phantom{.0} & $-0.0010$ \\
9191.8081(10) & II & 0.5 & 0.0011 \\
\hline
\end{tabular}
\end{center}
\end{table}

\IBVSfig{10cm}{5145-f1.eps}{Finding chart of V902 Sgr, VAR, the
comparison, COMP, and the check star, CHK}

\IBVSfig{10cm}{5145-f2.eps}{$B$, $V$, $R$ standard magnitude light
curves as defined by the individual observations}

\IBVSfig{10cm}{5145-f3.eps}{$B$, $V$, $B-V$ normalized flux light
curves, and computed light curves for V902 Sgr}

\IBVSfig{10cm}{5145-f4.eps}{$V$, $R$, $V-R$ normalized flux light
curves, and computed light curves for V902 Sgr}

\vskip-44pt
Four mean epochs of minimum light were determined from two primary and
secondary eclipses using the bisection of chords method. These
precision epochs of minimum light are given in Table 1 along with their
standard errors shown in parentheses. A linear ephemeris was
calculated using our timings, Ferwerda's (1943) and one time from
GCVS:
$$
{\rm J.D.~Hel.~Min~I} = 2449191\fday6599(84) + 0.29394574 (17) \times E.
$$
The standardized $BVR_c$ magnitude light curves and the $B-V$ and the
$V-R_c$ color curves of the variable are shown as Figure 2 as
calculated from the differential magnitudes (VAR${} - {}$COMP) versus
phase. The probably errors of a single observation were $\sim 1$\% in
$B$, $V$, and $R$. The photometric spectral types of the dwarf comparison
[$V = 13.95(2)$, $B-V = 0.85(2)$, $V-I_c = 0.88(3)$, $R_c-I_c =
0.41(2)$, $E(B-V) = 0.06$] and check stars [$V = 13.66(3)$, $B-V =
0.81(2)$, $V-I_c = 0.87(1)$, $R_c-I_c = 0.42(2)$] are K$0\pm0.3$ and
K$0\pm1$, respectively. The spectral type of the variable lies in the
K4 to G4 range, averaging about G9V\@. The $V$ magnitude range for the
variable is 13.81(1)--14.18(2). These curves have been solved using
the Wilson Code (Wilson 1994, 1990, Wilson \& Devinney 1971). These
yielded excellent fits to these asymmetric curves. The final parameters
include $m_2/m_1 = 0.1199(3)$, fill-out 43(3)\%, $T_2-T_1 =
93(5)$~K\@. The curves were dominated by two spot regions, a hot spot on
the secondary, less massive component, with a $T$ factor of 1.186(6) and
a radius of 30.8(6) degrees and a cool spot on the primary with a $T$
factor of 0.927(1) and a radius of 28.0(2) degrees. The colatitudes
were 129(1) and 104(1) degrees respectively. The solutions are shown in
Figure 3 and 4 overlying the normalized flux curves. The early
analysis of this binary was done as an undergraduate physics research
project by SFC.

{\it Acknowledgement.} This research was partially supported by a grant
from NASA administered by the American Astronomical Society.

\references

Ben, A.J., 1943, {\it AN}, {\bf 11}, No.\ 3 

Ferwerda, J.G., 1941, Leiden thesis

Ferwerda, J.G., 1943, {\it BAIN}, {\bf IX}, {\bf 354}, 337 

K.K. Kwee, 1962, {\it Leiden Ann.}, {\bf 22(1)} 

Wilson, R. E. \& Devinney, E. J. 1971, {\it ApJ}, {\bf 166}, 605

Wilson, R. E., 1990, {\it ApJ}, {\bf 356}, 613

Wilson, R. E., 1994, {\it PASP}, {\bf 106}, 921

\endreferences


%from IBVS 5259
\IBVSerratum{5145}


In IBVS 5145 the reference to the paper ``Ben, A.J., 1943, {\it AN}, {\bf 11}, No. 3"
is erroneous. The correct reference is:

\bigskip
\parindent=-1cm \leftskip 1cm

 P. Guthnick, H. Schneller, 1944, {\it Astronomische Abhandlungen
  (Erg\"anzungshefte zu den Astronomischen Nachrichten)}, {\bf 11}, 3.

\parindent=15.0pt \leftskip -0cm

\bigskip
\bigskip

The Editors

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