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\IBVShead{4787}{3 November 1999}

\IBVStitletl{CCD Observation of the 1999 September Outburst}{ of TY V\lowercase{ul}}

\IBVSauth{T.~Kato, M.~Uemura}

\IBVSinsto{Dept.\ of Astronomy, Kyoto University, Kyoto 606-8502, Japan,\\ \indent e-mail: tkato@kusastro.kyoto-u.ac.jp, uemura@kusastro.kyoto-u.ac.jp}

\SIMBADobj{TY Vul}
\IBVStyp{ UG }
\IBVSkey{Dwarf novae - photometry}

\begintext

TY Vul is a rather obscure dwarf nova. Downes et al.\ (1997) presented
an identification based on the chart by Meinunger (1980). This identification suggests a very faint quiescent counterpart. Bond (1978) reported the spectral type of F8:, which apparently refers to the close companion described below.

Schmeer (1999a,b) reported the apparently first-ever outburst since
the study by Meinunger (1980). The reported maximum magnitude was $V=14.6$ on 1999 September~1. We started time-resolved CCD photometry upon this alert. The observations were done using an unfiltered ST-7 camera attached to the Meade 25-cm Schmidt-Cassegrain telescope. The exposure time was 30~s. The images were dark-subtracted, flat-fielded, and analyzed using the Java$^{\rm TM}$-based PSF photometry package developed by one of the authors (TK). The relative fluxes of the variable were measured against GSC 2174.1749 (USNO $r$-magnitude 11.9), whose constancy was confirmed by comparison with GSC 2174.319 (USNO $r$-magnitude 12.5). The astrometry of the outbursting object based on GSC 1.1 gave 20\hr 41\mm 43\fsec 89, +25\deg 35\arcm 08\farcs 6 (J2000.0), which agrees with the cataloged position of USNO 1125.16991372 (20\hr 41\mm 43\fsec 82, +25\deg 35\arcm 08\farcs 9, J2000.0). However, Schmeer (private communication) pointed out that TY Vul and USNO 1125.16991372 make a close pair on his CCD image. The astrometric coincidence in our measurement may have partly been caused by the distortion of the GSC 1.1 astrometric grid of this region.

TY Vul and the companion being unresolved on our CCD images, our
measurements give the combined light of the two stars. Table~1 and Figure~1 show the nightly averaged fluxes.

TY Vul rapidly faded on the night of 1999 September 2 (JD 2451424), and the combined flux was virtually constant after September 4 (JD 2451426). We determined the averaged flux, as representing the flux of the companion, between the September 5 and 12 observations and subtracted it from the outburst observations. The rate of decline determined from the first two nights' observations was 1.1 mag d$^{-1}$.

Figure 2 shows the enlarged light curve (combined light) on 1999
September 2. A general decline consistent with the above rate was observed
during the run, but no apparent light modulation was detected.

\IBVSfig{10cm}{4787-f1.ps}{Light curve of TY Vul}

\IBVSfig{10cm}{4787-f2.ps}{Light curve of TY Vul on 1999 September 2}

\begin{table}
\begin{center}
\caption{Nightly averaged fluxes of TY Vul}
\begin{tabular}{ccccc}
\hline
JD start$^a$ & JD end$^a$ & mean flux$^b$ & error$^c$ & $N^d$ \\
\hline
51424.101 & 51424.302 & 0.0582 & 0.0009 & 351 \\
51426.102 & 51426.138 & 0.0277 & 0.0009 & 50  \\
51426.978 & 51427.038 & 0.0230 & 0.0055 & 48  \\
51430.103 & 51430.126 & 0.0262 & 0.0012 & 60  \\
51431.046 & 51431.090 & 0.0235 & 0.0019 & 61  \\
51432.961 & 51432.974 & 0.0171 & 0.0042 & 32  \\
51433.992 & 51433.996 & 0.0263 & 0.0073 & 10  \\
\hline
\end{tabular}

\begin{tabular}{r@{ }l}
$^a$ & ${\rm JD}-2400000$ \\
$^b$ & Combined flux (TY Vul and companion) \\
     & relative to GSC 2174.1749 \\
$^c$ & Standard error of nightly average \\
$^d$ & Number of frames \\
\end{tabular}
\end{center}
\end{table}

The relatively large rate of decline, the lack of light modulation and the large outburst amplitude suggest the present outburst may be a normal outburst of an SU UMa-type dwarf nova.

\bigskip

This work is partly supported by the Grant-in-Aid for Scientific Research
(10740095) of the Japanese Ministry of Education, Science, Culture.

\references

Bond, H. E., 1978, {\it PASP}, {\bf 90}, 526

Downes, R., Webbink, R. F, Shara, M. M., 1997, {\it PASP}, {\bf 109}, 345

Meinunger, I., 1980, {\it Ver\"{o}ff.\ Sternwarte Sonneberg}, {\bf 9}, 197

Schmeer, P., 1999a, {\it vsnet-alert circulation}, No.\ 3434, \\
(http://www.kusastro.kyoto-u.ac.jp/vsnet/Mail/vsnet-alert/msg03434.html).

Schmeer, P., 1999b, {\it vsnet-alert circulation}, No.\ 3435, \\
(http://www.kusastro.kyoto-u.ac.jp/vsnet/Mail/vsnet-alert/msg03435.html).

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