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\IBVShead{4602}{17 June 1998}

\IBVStitletl{OBSERVATION OF SUPERHUMPS IN SS UM\lowercase{i}}

\IBVSauth{T.~Kato$^1$, Y.~Lipkin$^2$, A.~Retter$^{2,3}$, E.~Leibowitz$^2$}
\vskip 5mm

\IBVSinst{Dept.\ of Astronomy, Kyoto University, Kyoto 606-8502, Japan,
          e-mail: tkato@kusastro.kyoto-u.ac.jp}
\IBVSinst{Wise Observatory and Department of Astronomy, Tel Aviv University,
          Ramat Aviv, Tel Aviv 69978, Israel
          e-mail: yiftah,elia@wise.tau.ac.il}
\IBVSinst{Present address: Physics Department, Keele University,
          Keele, Staffordshire, ST5 5BG, U.K.
          e-mail: ar@astro.keele.ac.uk}

\SIMBADobj{SS UMi}
\IBVStyp{ dwarf nova }
\IBVSkey{photometry}


\begintext

   The dwarf nova SS UMi was discovered as an optical counterpart of
Einstein IPC source E1551+718 (Mason et al.\ 1982).  Mason et al.\ (1982)
reported the system to be varying between $V\sim 13$ and $V\sim 17$.  The object
was independently detected as an ultraviolet-excess object, PG 1551+719
(Green et al.\ 1982).  Richter (1989) examined 4180 Sonneberg plates and
suggested the SS Cyg-type classification based on his finding that faint
maxima often last longer than bright maxima.  The insufficient detection
limit of Richter (1989), however, made the result inconclusive.  The
dwarf nova subtype was not certain -- while Andronov (1986) suggested the
127 min periodicity from photographic observations taken during quiescence, CCD photometry by Udalski
(1990) suggested a longer (6.8 or 9.5 h) period -- until the discovery
of superhumps by Chen et al.\ (1991).  The superhump period (101 min)
obtained by Chen et al.\ (1991) being only based on peak separations from
sparsely distributed short runs, it needs to be refined, especially in
view of the comparison with the orbital period (97.6 min) obtained by
radial velocity study (Thorstensen et al.\ 1996).




   The CCD observations were done using the 1.0-m telescope at the Wise
Observatory during the 1998 April superoutburst detected and communicated
on April 4 by Hanson (1998).  The log of observations is summarized in
Table 1.  Our observation started four days after the onset of the
superoutburst, and continued till its end.  We used a {\it V} filter;
the exposure time was 240 s.  We used four local comparison stars (C1
to C4 in Figure 1), whose non-variability was checked down to
0.02--0.05 mag (1-sigma error) by inter-comparison.  The magnitudes
of SS UMi are given relative to C1, whose magnitude we determined as
{\it V}=15.71.  The typical estimated error of each observation is 0.03
mag.  We applied heliocentric corrections to the observed times before
analysis.

Figure 2 presents the whole light curve, showing a characteristic decline
from superoutburst.  Superimposed superhumps are already apparent.


\vfil\eject

\begin{table}
\caption{Journal of observations}
\vskip 2mm
\begin{center}
\begin{tabular}{cccccc}
\hline
Year & Month  & Start (UT) &  End (UT) & JD$-$2400000$^{\star}$ & N$^{\star\star}$ \\
\hline
1998 & April  &   8.736    &    8.989  &  50912.236 & 87 \\
     &        &   9.794    &    9.990  &  50913.294 & 52 \\
     &        &  12.764    &   12.998  &  50916.264 & 60 \\
     &        &  13.752    &   13.989  &  50917.252 & 67 \\
\hline
\multicolumn{6}{l}{$^{\star}$: start of observation, $^{\star\star}$: number of observations} 
\end{tabular}
\end{center}
\end{table}



\IBVSfig{7cm}{4602-f1.ps}{Comparison stars}



\IBVSfig{7cm}{4602-f2.ps}{The entire light curve}

\vfil\eject

For the the first two nights' data during the superoutburst plateau
stage, we applied the Phase Dispersion Minimization (PDM) method
(Stellingwerf 1978), after removing the linear trend of decline.  The
resultant theta diagram displayed in Figure 3.  The best superhump
period is 0.0699 $\pm$ 0.0003 day, which is 3.1 $\pm$ 0.4 \% longer
than the orbital period (Thorstensen et al.\ 1996).

\IBVSfig{7cm}{4602-f3.ps}{Period analysis for 1998 April 8 and 9}
  

 Similar analysis was applied for the latter two nights.  The best obtained
period from the theta diagram (Figure 4) is 0.0677 $\pm$ 0.0003 day, which
exactly agrees with the orbital period within the estimated error.
The result implies that orbital signals predominate rather than late
superhumps at the very terminal stage of a superoutburst of this star.

\IBVSfig{7cm}{4602-f4.ps}{Period analysis for 1998 April 12 and 13}

\vfil\eject

   Figure 5 represents phase-averaged light curves of the first two nights
(upper panel) and of the last two nights (lower panel).  While the former
shows a typical profile of a superhump, the latter, folded by the orbital
period, shows a broader hump-like feature.

\IBVSfig{7cm}{4602-f5.ps}{Averaged light curves}


   
\references

Andronov, I. L. 1986, {\it Astron.\ Tsirk}, {\bf 1432}, 7

Chen, J.-S., Liu, X.-W., Wei, M.-Z. 1990, {\it A\&A}, {\bf 242}, 397

Green, R. F., Ferguson, D. H., Liebert, J. 1982, {\it PASP}, {\bf 94}, 560

Hanson, G. 1998, {\it vsnet-obs circular}, No.\,10949

Mason, K. O., Reichert, G. A., Bowyer, S., Thorstensen, J. R. 1982,
           {\it PASP}, {\bf 94}, 521

Richter, G. A. 1989, {\it AN}, {\bf 310}, 143

Stellingwerf, R. F. 1978, {\it ApJ}, {\bf 224}, 953

Thorstensen, J. R., Patterson, J. O., Shambrook, A., Thomas, G. 1996,
           {\it PASP}, {\bf 108}, 73

Udalski, A. 1990, {\it IBVS}, No.\,3425

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