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\input ibvs2.sty

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\IBVShead{4821}{22 December 1999}

\IBVStitle{HD 205798: A New Low-Amplitude Variable Star in Cygnus}

\IBVSauth{TORRES, G.; LATHAM, D.W.; STEFANIK, R.P.; MADER, J.}

\IBVSinsto{Harvard-Smithsonian Center for Astrophysics, 60 Garden St.,
Cambridge, MA 02138, USA,\\ \indent e-mail: gtorres@cfa.harvard.edu}

\SIMBADobjAlias{HD 205798}{HIP 106708}
\IBVStyp{ ELL }
\IBVSkey{photometry}
\IBVSabs{We report the discovery of the photometric variability of HD~205798 in}
\IBVSabs{the HIPPARCOS epoch photometry, which has gone unnoticed so far. This}
\IBVSabs{discovery was prompted by our finding that the star is a spectroscopic}
\IBVSabs{binary with a period of 0.6563 days.}

\begintext

The star HD~205798 (also HIP~106708; BD$+$33\deg4307; $\alpha =
21\hr36\mm53\fsec8$, $\delta = +33\deg43\arcm06\arcs$, J2000;
Sp.\ type F0; $V = 8.04$) is one of nearly 700 objects in a large
spectroscopic survey of F stars carried out at the Center for
Astrophysics.  The purpose of this survey is to determine their
kinematical properties and investigate various issues related to the
formation and evolution of the galactic disk (Nordstr\"om et al.\
1997). We have continued to observe the velocity variables in the
sample in order to study the frequency and orbital characteristics of
spectroscopic binaries among the early F stars. 

\IBVS2fig{7.5cm}{4821-f1.ps}{Spectroscopic orbital solution for\\ HD~205798.}{4821-f2.ps}{HIPPARCOS photometry for\\ HD~205798 phased with
the ephemeris above.}

Early in the course of this work HD~205798 was found to be
double-lined, and a preliminary mass ratio and center-of-mass velocity
were reported by Nordstr\"om et al.\ (1997). Only recently with many
more observations have we been able to determine the period, which is
0.6563 days. The radial velocity measurements and our orbital solution
are shown in Figure~1, and the preliminary elements are listed in
Table~1. The small minimum masses $M \sin^3 i$ indicate that the
inclination angle is fairly low if we assume that the primary
component has a mass typical for an F star. Thus light variations, if
any, are expected to be small. 

\begin{table}
\caption{Spectroscopic orbital solution for HD~205798.}
\begin{center}
\begin{tabular}{lc}
\hline
Parameter & Value \\
\hline
$P$ (days)                   & 0.65627696~$\pm$~0.00000053 \\
$\gamma$ (km~s$^{-1}$)       & $-$23.53~$\pm$~0.21\phantom{$-2$} \\
$K_A$ (km~s$^{-1}$)          & 17.58~$\pm$~0.29\phantom{1} \\
$K_B$ (km~s$^{-1}$)          & 82.24~$\pm$~1.02\phantom{8} \\
$e$                          & 0~(fixed) \\
$T_I$ (HJD$-$2,400,000)      & 50\,576.8561~$\pm$~0.0012\phantom{50\,45} \\
\noalign{\vskip 10pt}
$a_A \sin i$ ($10^6$~km)     & 0.1587~$\pm$~0.0027 \\
$a_B \sin i$ ($10^6$~km)     & 0.7422~$\pm$~0.0094 \\
$M_A \sin^3 i$ (M$_{\odot}$) & 0.05571~$\pm$~0.00190 \\
$M_B \sin^3 i$ (M$_{\odot}$) & 0.01191~$\pm$~0.00037 \\
$q\equiv M_B/M_A$            & 0.2138~$\pm$~0.0045 \\
\noalign{\vskip 10pt}
$N$                          & 67 \\
Time span (days)             & 4435 \\
$\sigma_A$ (km~s$^{-1}$)     & 1.8 \\
$\sigma_B$ (km~s$^{-1}$)     & 6.4 \\
\hline
\end{tabular}
\end{center}
\end{table}

The ephemeris for the primary minimum derived from our spectroscopic
solution is
$$
T_I~{\rm (HJD)} = 2\,450\,576.8561 (12) + 0.656\,276\,96 (53) \cdot E.
$$

The HIPPARCOS catalog (ESA 1997) does not flag this object as a
photometric variable. The median apparent brightness is listed as $H_p
= 8.1216$, with a scatter of 0.018 mag. However, when the epoch
photometry is phased with the above ephemeris the variations become
quite obvious (Figure~2).  There is a noticeable decrease in
brightness at phase 0.0, and a somewhat shallower secondary minimum at
phase 0.5, although the variation seems more or less continuous, as in
the ellipsoidal variables. The depth of the primary eclipse is only
about 0.05 mag. 

The precise nature of the system is as yet unclear.  Further
photometric observations in standard filters are needed for a full
solution of the light curve.  These observations are already underway,
along with a more definitive analysis of the spectroscopic material. 

\references

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

Nordstr\"om, B., Stefanik, R.\ P., Latham, D.\ W., \& Andersen, J.
1997, {\it A\&AS}, {\bf 126}, 21 % \BIBCODE{1997A&AS..126...21N}

\endreferences

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