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\IBVShead{4640}{22 October 1998}

\IBVStitletl{XY Bootis --}{ a W UM\lowercase{a}-star with extreme rate of period change}

\IBVSauth{Petr MOL\'IK,  Marek WOLF}

\IBVSinsto{Astronomical Institute, Charles University Prague,
          CZ-180 00 Praha 8, V Hole\v{s}ovi\v{c}k\'ach 2,
          Czech Republic, e-mail: P.Molik@vupp.cz}

\SIMBADobj{XY Boo}
\IBVStyp{ EW }
\IBVSkey{period study}

\begintext

XY Boo (= BD+20\deg 2874 = GSC 1466.0122 = HIP 67431 = 
FL 1632 = AN 21.1935 = P 3643;
$\alpha = 13\hr 49\mm
11\fsec 6$,
$\delta = +20\deg 11\arcm 24\farcs 5$, J2000;
$V_{max}$ = 10.3 mag; Sp.\ F5V)
was discovered as a variable star by Hoffmeister (1935). Tsesevich (1950, 1954)
observed this star visually, classified it as an eclipsing binary of
W UMa-type and determined the period of 0.31264 day. This period appeared
to be discordant with the first photoelectric observations of XY Boo
by Hinderer (1960). Therefore Wood (1965) reanalysed Hinderer's
photoelectric data and found a new period of 0.37054 day. Binnendijk (1971)
published six photoelectric minima timings of XY~Boo and revealed
remarkable increase in the period. This led him to the calculation of the
new linear ephemeris
\begin{equation}
\hbox{Pri.\ Min.\ = HJD} 2440389.7321 + 0\fday 37055251 \times  E  \\
\end{equation}
valid after 1960. Winkler (1977) observed XY~Boo photoelectrically in
1976 and derived three times of minimum light. He confirmed the long-term
increase in orbital period of this binary system. He also analysed his
own and Binnendijk's photoelectric data using the Wilson--Dewinney code
to determine geometric parameters of XY~Boo. The photometric mass ratio
$q$ = 0.18 calculated by Winkler was confirmed spectroscopically by
McLean and Hilditch (1983) who obtained $q$ = 0.16.

Awadalla and Yamasaki (1984) gave two photoelectric times of minima and
confirmed the period increase reported by earlier authors. Since that
time, more than twenty other photoelectric minima timings have been
obtained by Zhang et al.\ (1991), Agerer (1993, 1994), Diethelm (1994) and 
Kleikamp (Agerer and H\"ubscher, 1996). However, no detailed period study 
of XY~Boo has been published so far.

We performed our new CCD photometry of XY~Boo
during the night of May 8, 1998 at the Ond\v{r}ejov Observatory,
Czech Republic. A 65-cm telescope and the CCD camera SBIG ST-8
with standard $R$ Cousins filter were used.
Altogether 175 measurements in $R$ have been obtained (Fig.\ 1)\@.\IBVSfm\ 
The stars GSC 1466.0088 ($V$ = 10.2 mag) and GSC 1466.0010
($V$ = 10.2 mag) on the same frame as the variable
served as a comparison and check stars, respectively. These measurements
yielded one moment of secondary minimum.

\IBVSfoot{The table of
observational data in ASCII format is available as the 4640-t2.txt file
together with the electronic version of the Bulletin.}

In addition, four new times of minimum light based on the Hipparcos
photometric data were derived. They are included in Table 1 together 
with the given Hipparcos epoch and all other times of minimum 
of XY~Boo available in the literature.
A total of 43 moments of minimum spanning the interval 1944$-$1998
were incorporated in our analysis. Moments of minima denoted in Table~1 
as Nos 14 and 15 were excluded due to large deviation. After calculating 
O$-$C$_1$ residuals with respect to
the linear ephemeris (1) (see the fourth column in Table 1) it became
evident that the period increase of XY~Boo discovered by Binnendijk (1971)
still continues. We found out that the description of the long-term course 
of O$-$C residuals by a parabola is well substantiated even if the period 
increase has not been strictly continual and a series of discrete period 
changes took place within the 54 year interval. Consequently, using 
the method of least squares we calculated the following light elements 
with a quadratic term
\begin{equation}
\begin{array}{r@{}r@{}r@{}l}
\hbox{Pri.\ Min.\ = HJD} 2440389.7345 & {} + 0.3705526 & {} \times E + 3.10 & {} \times 10^{-10} \times E^2, \\
\pm 6 & \pm 4 & \pm 8
\end{array}
\end{equation}
which can serve also for the prediction of future times of minima.
The O$-$C$_2$ residuals calculated with respect to the quadratic elements are
given in the fifth column of Table 1. The O$-$C residuals for 43 times
of minimum with respect to the linear ephemeris (1) are shown in Figure 2.
The non-linear fit corresponding to the elements (2) is plotted as
a continuous curve.
\vskip-2mm
\IBVSfig{7cm}{4640-f1.ps}{Differential $R$ light curve
of XY~Boo obtained at JD 2450942.}
\vskip-2mm

The period increase by $6.2 \times 10^{-10}$ day/cycle or $6.12 \times 10^{-7}$
day/year or 5.3 seconds/century resulting from the elements (2) is 
extraordinarily large for a W~UMa-star. The highest rates of similar 
long-term period increases in W UMa-stars previously known to us were 
3.1 seconds/century in UZ~Leo (see Heged\"us \& J\"ager 1992) and 
2.7 seconds/century in V839~Oph (see Wolf et al.\ 1996).
Because the long-term increase in orbital period is usually
explained by mass transfer from the secondary to the primary component of
given binary we calculated the value of this supposed mass transfer for
XY~Boo. Adopting the parameters given for this system by Maceroni and 
Van't Veer (1996) we obtained the value of $1.34 \times 10^{-7}$ solar masses 
per year.


\begin{table}
\caption{The times of minimum of XY~Boo}
{\normalsize
\begin{center}
\begin{tabular}{rlrrrcc}
\hline
No. & JD hel    & Epoch  & O$-$C$_1$ & O$-$C$_2$ & Method & Reference \\
    & 2400000+  &        & [days]      & [days]      & Filter & \\
\hline
1 & 31215.386 & $-$24759.0 & 0.164\phantom{0} & $-$0.027\phantom{0} & v & Tsesevich, 1957 \\
2  & 35622.4578 & $-$12865.5 &    0.0690 &    0.0165 & e     & Hinderer, 1960 \\
3  & 35627.4596 & $-$12852.0 &    0.0684 &    0.0159 & e     & Hinderer, 1960 \\
4  & 39950.8121 & $-$1184.5 & $-$0.0006 & $-$0.0033 & e     & Binnendijk, 1971 \\
5  & 39951.9243 & $-$1181.5 & $-$0.0000 & $-$0.0027 & e     & Binnendijk, 1971 \\
6  & 39953.7763 & $-$1176.5 & $-$0.0008 & $-$0.0035 & e     & Binnendijk, 1971 \\
7  & 39953.9626 & $-$1176.0 &    0.0003 & $-$0.0025 & e     & Binnendijk, 1971 \\
8  & 40298.9470 & $-$245.0 &    0.0003 & $-$0.0021 & e     & Binnendijk, 1971 \\
9  & 40389.7319 &  0.0 & $-$0.0002 & $-$0.0026 & e     & Binnendijk, 1971 \\
10 & 42183.400  & 4840.5 & 0.009\phantom{0} & $-$0.002\phantom{0} & e   & Pohl \& Kizilirmak, 1975 \\
11 & 42569.7091 &     5883.0 &    0.0166 &    0.0029 & e     & Winkler, 1977 \\
12 & 42577.6745 &     5904.5 &    0.0151 &    0.0014 & e     & Winkler, 1977 \\
13 & 42582.6769 &     5918.0 &    0.0150 &    0.0013 & e     & Winkler, 1977 \\
14 & 44691.386  &    11608.5 &    0.095\phantom{0} &    0.050\phantom{0} & e     & Diethelm, 1981a \\
15 & 44716.390  &    11676.0 &    0.087\phantom{0} &    0.041\phantom{0} & e     & Diethelm, 1981b \\
16 & 45131.3793 & 12796.0 & 0.0573 & 0.0030 & e & Awadalla \& Yamasaki, 1984 \\
17 & 45132.3056 & 12798.5 & 0.0572 & 0.0029 & e & Awadalla \& Yamasaki, 1984 \\
18 & 46903.4115 &    17578.0 &    0.1074 &    0.0076 & e     & Diethelm, 1987 \\
19 & 47288.4252 &    18617.0 & 0.1170 &    0.0055 & e     & Diethelm, 1988 \\
20 & 48132.2036 &  20894.0 &  0.1478 &    0.0081 & e H   & this paper, Hip. \\
21 & 48132.3941 &  20894.5 &  0.1525 &    0.0129 & e H   & this paper, Hip. \\
22 & 48334.3320 & 21439.5 & 0.1394 & $-$0.0075 & e V   & Zhang et al., 1991 \\
23 & 48334.3319 &  21439.5 &  0.1393 & $-$0.0076 & e B & Zhang et al., 1991 \\
24 & 48335.2623 &  21442.0 &  0.1433 & $-$0.0036 & e V   & Zhang et al., 1991 \\
25 & 48335.2624 &  21442.0 &  0.1434 & $-$0.0035 & e B   & Zhang et al., 1991 \\
26 & 48336.3700 &  21445.0 &  0.1393 & $-$0.0076 & e V   & Zhang et al., 1991 \\
27 & 48336.3704 &  21445.0 &  0.1397 & $-$0.0072 & e B   & Zhang et al., 1991 \\
28 & 48363.2365 &  21517.5 &  0.1408 & $-$0.0071 & e V   & Zhang et al., 1991 \\
29 & 48363.2360 &  21517.5 &  0.1403 & $-$0.0076 & e B   & Zhang et al., 1991 \\
30 & 48364.1655 &  21520.0 &  0.1434 & $-$0.0045 & e V   & Zhang et al., 1991 \\
31 & 48364.1660 &  21520.0 &  0.1439 & $-$0.0040 & e B   & Zhang et al., 1991 \\
32 & 48500.1730 &    21887.0 &  0.1581 &    0.0052 & e H   & Hipparcos epoch\\
33 & 48774.3912 &    22627.0 &  0.1673 &    0.0041 & e H   & this paper, Hip. \\
34 & 48774.5807 &    22627.5 &   0.1720 &    0.0088 & e H   & this paper, Hip. \\
35 & 49130.5078 &    23588.0 &    0.1831 &    0.0061 & e B   & Agerer, 1993 \\
36 & 49130.5078 &    23588.0 &    0.1831 &    0.0061 & e V   & Agerer, 1993 \\
37 & 49154.4059 &    23652.5 &    0.1806 &    0.0026 & e V   & Agerer, 1994 \\
38 & 49154.4091 &    23652.5 &    0.1838 &    0.0058 & e B   & Agerer, 1994 \\
39 & 49166.4481 &    23685.0 &    0.1798 &    0.0014 & e V   & Agerer, 1994 \\
40 & 49166.4499 &    23685.0 &    0.1816 &    0.0032 & e B   & Agerer, 1994 \\
41 & 49504.4082 &    24597.0 &    0.1960 &    0.0038 & e B   & Agerer, 1994 \\
42 & 49504.4096 &    24597.0 &    0.1974 &    0.0052 & e V   & Agerer, 1994 \\
43 & 49511.4497 &    24616.0 &    0.1970 &    0.0046 & e B   & Diethelm, 1994 \\
44 & 49859.4101 & 25555.0 & 0.2086 & 0.0015 & CCD & Agerer \& H\"ubscher, 1996 \\
45 & 50942.3992 &    28477.5 &    0.2580 &    0.0016 & CCD R & this paper \\
\hline
\multicolumn{7}{l}{Note: in the 6th column ``v'' means visual, ``e'' photoelectric observation.}
\end{tabular}
\end{center}
}
\end{table}




XY~Boo, as noticed by Kaluzny (1985), has one of the shortest
orbital periods for given spectral type (F5). In its position in the
period-colour diagram one would expect an unevolved system with components
having similar masses but XY~Boo seems to be evolved and the mass ratio
of its components is approximately 6:1. This makes the explanation of
evolution of this system difficult. Further observations are needed
to decide about the true nature of period changes as well as
evolutionary status of this interesting system.

{\it Acknowledgement.}
We are thankful to Dr.\ Roger Diethelm, BBSAG,
for critical analysis of his previous measurements.


\IBVSfig{7cm}{4640-f2.ps}{O$-$C diagram of XY~Boo.}

\vskip-5mm
\references

Agerer F., 1993, {\it BAV Mitteilungen}, No.\,62

Agerer F., 1994, {\it BAV Mitteilungen}, No.\,68

Agerer F., H\"ubscher J., 1996, {\it Inform.\ Bull.\ Var.\ Stars}, No.\,4382

Awadalla N.S., Yamasaki A., 1984, {\it Astrophys.\ Space Sci.}, {\bf 107}, 347

Binnendijk L., 1971, {\it Astron.\ J.}, {\bf 76}, 923

Diethelm R., 1981a, {\it BBSAG Bull.}, No.\,53

Diethelm R., 1981b, {\it BBSAG Bull.}, No.\,54

Diethelm R., 1987, {\it BBSAG Bull.}, No.\,83

Diethelm R., 1988, {\it BBSAG Bull.}, No.\,88

Diethelm R., 1994, {\it BBSAG Bull.}, No.\,107

Heged\"us, T., J\"ager, Z., 1992, {\it Publ.\ Astron.\ Soc.\ Pacific}, {\bf 104}, 733

Hinderer F., 1960, {\it J. Obs.}, {\bf 43}, 161

Hoffmeister C., 1935, {\it Astron. Nachr.}, {\bf 255}, 401

Maceroni C., Van't Veer F., 1996, {\it Astron.\ Astrophys.}, {\bf 311}, 523

McLean B.J., Hilditch R.W., 1983, {\it Mon.\ Not.\ Roy.\ Astron.\ Soc.}, {\bf 203}, 1

Kaluzny J., 1985, {\it Acta Astron.}, {\bf 35}, 313

Pohl E., Kizilirmak A., 1975, {\it Inform.\ Bull.\ Var.\ Stars}, No.\,1053

Tsesevich V.P., 1950, {\it Astron.\ Circ.\ Kazan}, {\bf 100}, 18

Tsesevich V.P., 1954, {\it Odessa Izv.}, {\bf 4}, 137

Winkler L., 1977, {\it Astron.\ J.}, {\bf 82}, 648

Wolf M., \v{S}arounov\'a L., Mol\'{\i}k P., 1996, {\it Inform.\ Bull.\ Var.\ Stars}, No.\,4304

Wood R., 1965, {\it Observatory}, {\bf 85}, 258

Zhang Y., Qingyao L., Bi W., Shenhong G., 1991, {\it Inform.\ Bul.\ Var.\ Stars}, No.\,3662


\IBVSedata{4640-t2.txt}

\end{document}
