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\IBVShead{5104}{5 June 2001}

\IBVStitle{Development of Late Superhumps in YZ C\lowercase{nc}}

\IBVSauth{Kato, Taichi}

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

\SIMBADobj{YZ Cnc}
\IBVStyp{ UGSU }
\IBVSkey{dwarf nova, photometry}

\begintext

YZ Cnc is a well-known dwarf nova and is a prototype object
representing for a population of SU UMa-type systems with short
outburst recurrence times and long orbital periods.  However, little
photometric observation of superhumps had been done since its
identification as an SU UMa-type star (Patterson 1979).  We undertook
time-resolved CCD photometry during its superoutburst in 1994 January.

The observations were done on three successive nights between 1994
January 1 and 3, using a CCD camera (Thomson TH~7882, $576 \times 384$
pixels, on-chip $2 \times 2$ binning adopted) attached to the
Cassegrain focus of the 60-cm reflector (focal length${}=4.8$~m) at
Ouda Station, Kyoto University (Ohtani et al.\ 1992).  An interference
filter was used which had been designed to reproduce the Johnson $V$
band.  The exposure time was 60--120 s depending on the brightness of
the object.  The frames were first corrected for standard de-biasing
and flat fielding, and were then processed by a microcomputer-based
aperture photometry package developed by the author.  The magnitudes of
the object were determined relative to GSC 1939.1130 (GSC magnitude
13.4), but its constancy was not confirmed because of the lack of
suitable check stars in the same field.  Barycentric corrections to
observed times were applied before the following analysis.  Table 1
lists the log of observations, together with nightly averaged
magnitudes.

The light curve drawn from these data is presented in Figure 1.  The
light curve shows the declining portion from a superoutburst.
Superhumps were evident on the first night, but became more complex on
the next night, when the system entered the rapidly declining phase.
The period analysis over the entire, or selected, data sets does not
yield a coherent signal, because of the development of late superhumps
as described below.  So we used the primary superhump period of
$P=0\fday09204$ (Patterson 1979) for the following analysis.

Figure 2 shows the phase-averaged light curves of 1994 January 1 (upper
panel) and January 2 (lower panel).  The January 1 light curve clearly
shows typical superhumps, with a shoulder (secondary superhumps) on its
declining branch.  However, the phase of the maximum dramatically
changed by $\phi\sim0.3$--0.4 on the next night (lower panel).  The
newly appeared humps correspond to what are called ``late superhumps''
(Haefner et al.\ 1979), which is considered to reflect the modulation
of the precessing accretion disk properties at the stream impact point
(Hessman et al.\ 1992).  The clear appearance of late superhumps in YZ
Cnc may be consistent with its high mass-transfer rate.

\IBVSfig{10cm}{5104-f1.ps}{Light curve of the 1994 January superoutburst
of YZ Cnc}

\IBVSfig{10cm}{5104-f2.ps}{Phase-averaged light curve of YZ Cnc, assuming
the superhump period of 0\fday09204.  The origin of the phase is arbitrarily
taken as BJD 2449350}

\clearpage

\begin{table}
\begin{center}
\caption{Log of observations}
\begin{tabular}{ccrcr}
\hline
start$^a$ & end$^a$ & mean mag$^b$ & error$^c$ & $N^d$ \\
\hline
49354.211 & 49354.385 & $-0.800$ & 0.005 & 162 \\
49355.178 & 49355.340 &  0.285 & 0.014 &  94 \\
49356.213 & 49356.317 &  1.119 & 0.035 &  30 \\
\hline
\end{tabular}

\normalsize
\begin{tabular}{l@{ }l}
$^a$ & BJD${}-{}$2400000 \\
$^b$ & Magnitude relative to GSC 1939.1130 \\
$^c$ & Standard error of nightly average \\
$^d$ & Number of frames \\
\end{tabular}
\end{center}
\end{table}

\references

Haefner, R., Schoembs, R., Vogt, R., 1979, {\it A\&A}, {\bf 77}, 7

Hessman, F. V., Mantel, K.-H., Barwig, H., Schoembs, R., 1992, {\it
A\&A}, {\bf 263}, 147

Ohtani, H., Uesugi, A., Tomita, Y., Yoshida, M., Kosugi, G., Noumaru,
J., Araya, S., Ohta, K., 1992, {\it Memoirs of the Faculty of Science,
Kyoto University, Series A of Physics, Astrophysics, Geophysics and
Chemistry}, {\bf 38}, 167

Patterson, J., 1979, {\it AJ}, {\bf 84}, 804

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